diff --git a/src/core/IronPython.Modules/_ctypes/INativeType.cs b/src/core/IronPython.Modules/_ctypes/INativeType.cs
index a3a99f166..18d4834cd 100644
--- a/src/core/IronPython.Modules/_ctypes/INativeType.cs
+++ b/src/core/IronPython.Modules/_ctypes/INativeType.cs
@@ -50,7 +50,7 @@ int Alignment {
             /// Serializes the provided value into the specified address at the given
             /// offset.
             /// </summary>
-            object SetValue(MemoryHolder/*!*/ address, int offset, object value);
+            object? SetValue(MemoryHolder/*!*/ address, int offset, object value);
 
             /// <summary>
             /// Gets the .NET type which is used when calling or returning the value
@@ -68,12 +68,12 @@ int Alignment {
             /// Emits marshalling of an object from Python to native code.  This produces the
             /// native type from the Python type.
             /// </summary>
-            MarshalCleanup EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg/*!*/ argIndex, List<object>/*!*/ constantPool, int constantPoolArgument);
+            MarshalCleanup? EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg/*!*/ argIndex, List<object>/*!*/ constantPool, int constantPoolArgument);
 
             /// <summary>
             /// Emits marshalling from native code to Python code This produces the python type 
             /// from the native type.  This is used for return values and parameters 
-            /// to Python callable objects that are passed back out to native code.
+            /// to Python callable objects that are passed back out of native code.
             /// </summary>
             void EmitReverseMarshalling(ILGenerator/*!*/ method, LocalOrArg/*!*/ value, List<object>/*!*/ constantPool, int constantPoolArgument);
 
diff --git a/src/core/IronPython.Modules/_ctypes/StructType.cs b/src/core/IronPython.Modules/_ctypes/StructType.cs
index e4ffdafee..b37878ec2 100644
--- a/src/core/IronPython.Modules/_ctypes/StructType.cs
+++ b/src/core/IronPython.Modules/_ctypes/StructType.cs
@@ -2,15 +2,16 @@
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 #if FEATURE_CTYPES
 
 using System;
-using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Numerics;
 using System.Reflection.Emit;
-using System.Runtime.InteropServices;
 using System.Text;
 
 using Microsoft.Scripting;
@@ -19,6 +20,7 @@
 using IronPython.Runtime.Operations;
 using IronPython.Runtime.Types;
 
+
 namespace IronPython.Modules {
     /// <summary>
     /// Provides support for interop with native code from Python code.
@@ -30,15 +32,16 @@ public static partial class CTypes {
         /// </summary>
         [PythonType, PythonHidden]
         public class StructType : PythonType, INativeType {
-            internal Field[] _fields;
+            [DisallowNull]
+            internal Field[]? _fields;              // not null after type construction completes
             private int? _size, _alignment, _pack;
-            private static readonly Field[] _emptyFields = System.Array.Empty<Field>(); // fields were never initialized before a type was created
+            private static readonly Field[] _emptyFields = []; // fields were never initialized before a type was created
 
-            public StructType(CodeContext/*!*/ context, string name, PythonTuple bases, PythonDictionary members)
+            public StructType(CodeContext/*!*/ context, [NotNone] string name, [NotNone] PythonTuple bases, [NotNone] PythonDictionary members)
                 : base(context, name, bases, members) {
 
                 foreach (PythonType pt in ResolutionOrder) {
-                    StructType st = pt as StructType;
+                    StructType? st = pt as StructType;
                     if (st != this) {
                         st?.EnsureFinal();
                     }
@@ -71,11 +74,11 @@ private StructType(Type underlyingSystemType)
                 : base(underlyingSystemType) {
             }
 
-            public static ArrayType/*!*/ operator *(StructType type, int count) {
+            public static ArrayType/*!*/ operator *([NotNone] StructType type, int count) {
                 return MakeArrayType(type, count);
             }
 
-            public static ArrayType/*!*/ operator *(int count, StructType type) {
+            public static ArrayType/*!*/ operator *(int count, [NotNone] StructType type) {
                 return MakeArrayType(type, count);
             }
 
@@ -93,13 +96,13 @@ public _Structure from_address(CodeContext/*!*/ context, IntPtr ptr) {
                 return res;
             }
 
-            public _Structure from_buffer(CodeContext/*!*/ context, object/*?*/ data, int offset = 0) {
+            public _Structure from_buffer(CodeContext/*!*/ context, object? data, int offset = 0) {
                 _Structure res = (_Structure)CreateInstance(context);
                 res.InitializeFromBuffer(data, offset, ((INativeType)this).Size);
                 return res;
             }
 
-            public _Structure from_buffer_copy(CodeContext/*!*/ context, object/*?*/ data, int offset = 0) {
+            public _Structure from_buffer_copy(CodeContext/*!*/ context, object? data, int offset = 0) {
                 _Structure res = (_Structure)CreateInstance(context);
                 res.InitializeFromBufferCopy(data, offset, ((INativeType)this).Size);
                 return res;
@@ -110,19 +113,19 @@ public _Structure from_buffer_copy(CodeContext/*!*/ context, object/*?*/ data, i
             /// 
             /// Structures just return themselves.
             /// </summary>
-            public object from_param(object obj) {
+            public object from_param(object? obj) {
                 if (!Builtin.isinstance(obj, this)) {
-                    throw PythonOps.TypeError("expected {0} instance got {1}", Name, PythonOps.GetPythonTypeName(obj));
+                    throw PythonOps.TypeError("expected {0} instance, got {1}", Name, PythonOps.GetPythonTypeName(obj));
                 }
 
-                return obj;
+                return obj!;
             }
 
-            public object in_dll(object library, string name) {
+            public object in_dll(object? library, [NotNone] string name) {
                 throw new NotImplementedException("in dll");
             }
 
-            public new virtual void __setattr__(CodeContext/*!*/ context, string name, object value) {
+            public new virtual void __setattr__(CodeContext/*!*/ context, [NotNone] string name, object? value) {
                 if (name == "_fields_") {
                     lock (this) {
                         if (_fields != null) {
@@ -160,7 +163,7 @@ object INativeType.GetValue(MemoryHolder/*!*/ owner, object readingFrom, int off
                 return res;
             }
 
-            object INativeType.SetValue(MemoryHolder/*!*/ address, int offset, object value) {
+            object? INativeType.SetValue(MemoryHolder/*!*/ address, int offset, object value) {
                 try {
                     return SetValueInternal(address, offset, value);
                 } catch (ArgumentTypeException e) {
@@ -174,9 +177,9 @@ object INativeType.SetValue(MemoryHolder/*!*/ address, int offset, object value)
                 }
             }
 
-            internal object SetValueInternal(MemoryHolder address, int offset, object value) {
-                IList<object> init = value as IList<object>;
-                if (init != null) {
+            internal object? SetValueInternal(MemoryHolder address, int offset, object value) {
+                if (value is IList<object> init) {
+                    EnsureFinal();
                     if (init.Count > _fields.Length) {
                         throw PythonOps.TypeError("too many initializers");
                     }
@@ -184,14 +187,11 @@ internal object SetValueInternal(MemoryHolder address, int offset, object value)
                     for (int i = 0; i < init.Count; i++) {
                         _fields[i].SetValue(address, offset, init[i]);
                     }
+                } else if (value is CData data) {
+                    data.MemHolder.CopyTo(address, offset, data.Size);
+                    return data.MemHolder.EnsureObjects();
                 } else {
-                    CData data = value as CData;
-                    if (data != null) {
-                        data.MemHolder.CopyTo(address, offset, data.Size);
-                        return data.MemHolder.EnsureObjects();
-                    } else {
-                        throw new NotImplementedException("set value");
-                    }
+                    throw new NotImplementedException("set value");
                 }
                 return null;
             }
@@ -202,7 +202,7 @@ internal object SetValueInternal(MemoryHolder address, int offset, object value)
                 return GetMarshalTypeFromSize(_size.Value);
             }
 
-            MarshalCleanup INativeType.EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg argIndex, List<object>/*!*/ constantPool, int constantPoolArgument) {
+            MarshalCleanup? INativeType.EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg argIndex, List<object>/*!*/ constantPool, int constantPoolArgument) {
                 Type argumentType = argIndex.Type;
                 argIndex.Emit(method);
                 if (argumentType.IsValueType) {
@@ -212,8 +212,8 @@ MarshalCleanup INativeType.EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg a
                 method.Emit(OpCodes.Ldarg, constantPoolArgument);
                 method.Emit(OpCodes.Ldc_I4, constantPool.Count - 1);
                 method.Emit(OpCodes.Ldelem_Ref);
-                method.Emit(OpCodes.Call, typeof(ModuleOps).GetMethod(nameof(ModuleOps.CheckCDataType)));
-                method.Emit(OpCodes.Call, typeof(CData).GetProperty(nameof(CData.UnsafeAddress)).GetGetMethod());
+                method.Emit(OpCodes.Call, typeof(ModuleOps).GetMethod(nameof(ModuleOps.CheckCDataType))!);
+                method.Emit(OpCodes.Call, typeof(CData).GetProperty(nameof(CData.UnsafeAddress))!.GetGetMethod()!);
                 method.Emit(OpCodes.Ldobj, ((INativeType)this).GetNativeType());
                 return null;
             }
@@ -251,24 +251,24 @@ internal static PythonType MakeSystemType(Type underlyingSystemType) {
                 return PythonType.SetPythonType(underlyingSystemType, new StructType(underlyingSystemType));
             }
 
-            private void SetFields(object fields) {
+            [MemberNotNull(nameof(_fields), nameof(_size), nameof(_alignment))]
+            private void SetFields(object? fields) {
                 lock (this) {
-                    IList<object> list = GetFieldsList(fields);
+                    IList<object> fieldDefList = GetFieldsList(fields);
 
                     int? bitCount = null;
                     int? curBitCount = null;
-                    INativeType lastType = null;
+                    INativeType? lastType = null;
                     List<Field> allFields = GetBaseSizeAlignmentAndFields(out int size, out int alignment);
 
-                    IList<object> anonFields = GetAnonymousFields(this);
+                    IList<object>? anonFields = GetAnonymousFields(this);
 
-                    for (int fieldIndex = 0; fieldIndex < list.Count; fieldIndex++) {
-                        object o = list[fieldIndex];
-                        GetFieldInfo(this, o, out string fieldName, out INativeType cdata, out bitCount);
+                    foreach (object fieldDef in fieldDefList) {
+                        GetFieldInfo(this, fieldDef, out string fieldName, out INativeType cdata, out bitCount);
 
                         int prevSize = UpdateSizeAndAlignment(cdata, bitCount, lastType, ref size, ref alignment, ref curBitCount);
 
-                        Field newField = new Field(fieldName, cdata, prevSize, allFields.Count, bitCount, curBitCount - bitCount);
+                        var newField = new Field(fieldName, cdata, prevSize, allFields.Count, bitCount, curBitCount - bitCount);
                         allFields.Add(newField);
                         AddSlot(fieldName, newField);
 
@@ -282,16 +282,18 @@ private void SetFields(object fields) {
                     CheckAnonymousFields(allFields, anonFields);
 
                     if (bitCount != null) {
-                        size += lastType.Size;
+                        // incomplete last bitfield
+                        // bitCount not null implies at least one bitfield, so at least one iteration of the loop above
+                        size += lastType!.Size;
                     }
 
-                    _fields = allFields.ToArray();
+                    _fields = [..allFields];
                     _size = PythonStruct.Align(size, alignment);
                     _alignment = alignment;
                 }
             }
 
-            internal static void CheckAnonymousFields(List<Field> allFields, IList<object> anonFields) {
+            internal static void CheckAnonymousFields(List<Field> allFields, IList<object>? anonFields) {
                 if (anonFields != null) {
                     foreach (string s in anonFields) {
                         bool found = false;
@@ -309,9 +311,9 @@ internal static void CheckAnonymousFields(List<Field> allFields, IList<object> a
                 }
             }
 
-            internal static IList<object> GetAnonymousFields(PythonType type) {
+            internal static IList<object>? GetAnonymousFields(PythonType type) {
                 object anonymous;
-                IList<object> anonFields = null;
+                IList<object>? anonFields = null;
                 if (type.TryGetBoundAttr(type.Context.SharedContext, type, "_anonymous_", out anonymous)) {
                     anonFields = anonymous as IList<object>;
                     if (anonFields == null) {
@@ -323,16 +325,18 @@ internal static IList<object> GetAnonymousFields(PythonType type) {
 
             internal static void AddAnonymousFields(PythonType type, List<Field> allFields, INativeType cdata, Field newField) {
                 Field[] childFields;
-                if (cdata is StructType) {
-                    childFields = ((StructType)cdata)._fields;
-                } else if (cdata is UnionType) {
-                    childFields = ((UnionType)cdata)._fields;
+                if (cdata is StructType st) {
+                    st.EnsureFinal();
+                    childFields = st._fields;
+                } else if (cdata is UnionType un) {
+                    un.EnsureFinal();
+                    childFields = un._fields;
                 } else {
                     throw PythonOps.TypeError("anonymous field must be struct or union");
                 }
 
                 foreach (Field existingField in childFields) {
-                    Field anonField = new Field(
+                    var anonField = new Field(
                         existingField.FieldName,
                         existingField.NativeType,
                         checked(existingField.offset + newField.offset),
@@ -347,12 +351,12 @@ internal static void AddAnonymousFields(PythonType type, List<Field> allFields,
             private List<Field> GetBaseSizeAlignmentAndFields(out int size, out int alignment) {
                 size = 0;
                 alignment = 1;
-                List<Field> allFields = new List<Field>();
-                INativeType lastType = null;
+                List<Field> allFields = [];
+                INativeType? lastType = null;
                 int? totalBitCount = null;
                 foreach (PythonType pt in BaseTypes) {
-                    StructType st = pt as StructType;
-                    if (st != null) {
+                    if (pt is StructType st) {
+                        st.EnsureFinal();
                         foreach (Field f in st._fields) {
                             allFields.Add(f);
                             UpdateSizeAndAlignment(f.NativeType, f.BitCount, lastType, ref size, ref alignment, ref totalBitCount);
@@ -368,7 +372,8 @@ private List<Field> GetBaseSizeAlignmentAndFields(out int size, out int alignmen
                 return allFields;
             }
 
-            private int UpdateSizeAndAlignment(INativeType cdata, int? bitCount, INativeType lastType, ref int size, ref int alignment, ref int? totalBitCount) {
+            private int UpdateSizeAndAlignment(INativeType cdata, int? bitCount, INativeType? lastType, ref int size, ref int alignment, ref int? totalBitCount) {
+                Debug.Assert(totalBitCount == null || lastType != null); // lastType is null only on the first iteration, when totalBitCount is null as well
                 int prevSize = size;
                 if (bitCount != null) {
                     if (lastType != null && lastType.Size != cdata.Size) {
@@ -382,7 +387,7 @@ private int UpdateSizeAndAlignment(INativeType cdata, int? bitCount, INativeType
                         if ((bitCount + totalBitCount + 7) / 8 <= cdata.Size) {
                             totalBitCount = bitCount + totalBitCount;
                         } else {
-                            size += lastType.Size;
+                            size += lastType!.Size;
                             prevSize = size;
                             totalBitCount = bitCount;
                         }
@@ -391,7 +396,7 @@ private int UpdateSizeAndAlignment(INativeType cdata, int? bitCount, INativeType
                     }
                 } else {
                     if (totalBitCount != null) {
-                        size += lastType.Size;
+                        size += lastType!.Size;
                         prevSize = size;
                         totalBitCount = null;
                     }
@@ -411,6 +416,7 @@ private int UpdateSizeAndAlignment(INativeType cdata, int? bitCount, INativeType
                 return prevSize;
             }
 
+            [MemberNotNull(nameof(_fields), nameof(_size), nameof(_alignment))]
             internal void EnsureFinal() {
                 if (_fields == null) {
                     SetFields(PythonTuple.EMPTY);
@@ -419,6 +425,8 @@ internal void EnsureFinal() {
                         // track that we were initialized w/o fields.
                         _fields = _emptyFields;
                     }
+                } else if (_size == null || _alignment == null) {
+                    throw new InvalidOperationException("fields initialized w/o size or alignment");
                 }
             }
 
@@ -427,19 +435,22 @@ internal void EnsureFinal() {
             /// from all of our base classes.  If later new _fields_ are added we'll be
             /// initialized and these values will be replaced.
             /// </summary>
+            [MemberNotNull(nameof(_size), nameof(_alignment))]
             private void EnsureSizeAndAlignment() {
                 Debug.Assert(_size.HasValue == _alignment.HasValue);
-                // these are always iniitalized together
+                // these are always initialized together
                 if (_size == null) {
                     lock (this) {
                         if (_size == null) {
-                            int size, alignment;
-                            GetBaseSizeAlignmentAndFields(out size, out alignment);
+                            GetBaseSizeAlignmentAndFields(out int size, out int alignment);
                             _size = size;
                             _alignment = alignment;
                         }
                     }
                 }
+                if (_alignment == null) {
+                    throw new InvalidOperationException("size and alignment should always be initialized together");
+                }
             }
         }
     }
diff --git a/src/core/IronPython.Modules/_ctypes/UnionType.cs b/src/core/IronPython.Modules/_ctypes/UnionType.cs
index ce2e0c0b6..afbc95d64 100644
--- a/src/core/IronPython.Modules/_ctypes/UnionType.cs
+++ b/src/core/IronPython.Modules/_ctypes/UnionType.cs
@@ -2,19 +2,20 @@
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 #if FEATURE_CTYPES
 
 using System;
-using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection.Emit;
 
-using Microsoft.Scripting;
-
 using IronPython.Runtime;
 using IronPython.Runtime.Operations;
 using IronPython.Runtime.Types;
 
+
 namespace IronPython.Modules {
     /// <summary>
     /// Provides support for interop with native code from Python code.
@@ -25,19 +26,18 @@ public static partial class CTypes {
         /// </summary>
         [PythonType, PythonHidden]
         public class UnionType : PythonType, INativeType {
-            internal Field[] _fields;
+            internal Field[]? _fields;
             private int _size, _alignment;
 
-            public UnionType(CodeContext/*!*/ context, string name, PythonTuple bases, PythonDictionary members)
+            public UnionType(CodeContext/*!*/ context, [NotNone] string name, [NotNone] PythonTuple bases, [NotNone] PythonDictionary members)
                 : base(context, name, bases, members) {
 
-                object fields;
-                if (members.TryGetValue("_fields_", out fields)) {
+                if (members.TryGetValue("_fields_", out object fields)) {
                     SetFields(fields);
                 }
             }
 
-            public new void __setattr__(CodeContext/*!*/ context, string name, object value) {
+            public new void __setattr__(CodeContext/*!*/ context, [NotNone] string name, object? value) {
                 if (name == "_fields_") {
                     lock (this) {
                         if (_fields != null) {
@@ -58,7 +58,7 @@ private UnionType(Type underlyingSystemType)
             /// <summary>
             /// Converts an object into a function call parameter.
             /// </summary>
-            public object from_param(object obj) {
+            public object? from_param(object? obj) {
                 return null;
             }
 
@@ -66,21 +66,21 @@ internal static PythonType MakeSystemType(Type underlyingSystemType) {
                 return PythonType.SetPythonType(underlyingSystemType, new UnionType(underlyingSystemType));
             }
 
-            public static ArrayType/*!*/ operator *(UnionType type, int count) {
+            public static ArrayType/*!*/ operator *([NotNone] UnionType type, int count) {
                 return MakeArrayType(type, count);
             }
 
-            public static ArrayType/*!*/ operator *(int count, UnionType type) {
+            public static ArrayType/*!*/ operator *(int count, [NotNone] UnionType type) {
                 return MakeArrayType(type, count);
             }
 
-            public _Union from_buffer(CodeContext/*!*/ context, object/*?*/ data, int offset = 0) {
+            public _Union from_buffer(CodeContext/*!*/ context, object? data, int offset = 0) {
                 _Union res = (_Union)CreateInstance(context);
                 res.InitializeFromBuffer(data, offset, ((INativeType)this).Size);
                 return res;
             }
 
-            public _Union from_buffer_copy(CodeContext/*!*/ context, object/*?*/ data, int offset = 0) {
+            public _Union from_buffer_copy(CodeContext/*!*/ context, object? data, int offset = 0) {
                 _Union res = (_Union)CreateInstance(context);
                 res.InitializeFromBufferCopy(data, offset, ((INativeType)this).Size);
                 return res;
@@ -106,9 +106,9 @@ object INativeType.GetValue(MemoryHolder owner, object readingFrom, int offset,
                 return res;
             }
 
-            object INativeType.SetValue(MemoryHolder address, int offset, object value) {
-                IList<object> init = value as IList<object>;
-                if (init != null) {
+            object? INativeType.SetValue(MemoryHolder address, int offset, object value) {
+                if (value is IList<object> init) {
+                    EnsureFinal();
                     if (init.Count > _fields.Length) {
                         throw PythonOps.TypeError("too many initializers");
                     }
@@ -116,14 +116,11 @@ object INativeType.SetValue(MemoryHolder address, int offset, object value) {
                     for (int i = 0; i < init.Count; i++) {
                         _fields[i].SetValue(address, offset, init[i]);
                     }
+                } else if (value is CData data) {
+                    data.MemHolder.CopyTo(address, offset, data.Size);
+                    return data.MemHolder.EnsureObjects();
                 } else {
-                    CData data = value as CData;
-                    if (data != null) {
-                        data.MemHolder.CopyTo(address, offset, data.Size);
-                        return data.MemHolder.EnsureObjects();
-                    } else {
-                        throw new NotImplementedException("Union set value");
-                    }
+                    throw new NotImplementedException("Union set value");
                 }
                 return null;
             }
@@ -132,7 +129,7 @@ Type INativeType.GetNativeType() {
                 return GetMarshalTypeFromSize(_size);
             }
 
-            MarshalCleanup INativeType.EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg argIndex, List<object>/*!*/ constantPool, int constantPoolArgument) {
+            MarshalCleanup? INativeType.EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg argIndex, List<object>/*!*/ constantPool, int constantPoolArgument) {
                 Type argumentType = argIndex.Type;
                 argIndex.Emit(method);
                 if (argumentType.IsValueType) {
@@ -142,8 +139,8 @@ MarshalCleanup INativeType.EmitMarshalling(ILGenerator/*!*/ method, LocalOrArg a
                 method.Emit(OpCodes.Ldarg, constantPoolArgument);
                 method.Emit(OpCodes.Ldc_I4, constantPool.Count - 1);
                 method.Emit(OpCodes.Ldelem_Ref);
-                method.Emit(OpCodes.Call, typeof(ModuleOps).GetMethod(nameof(ModuleOps.CheckCDataType)));
-                method.Emit(OpCodes.Call, typeof(CData).GetProperty(nameof(CData.UnsafeAddress)).GetGetMethod());
+                method.Emit(OpCodes.Call, typeof(ModuleOps).GetMethod(nameof(ModuleOps.CheckCDataType))!);
+                method.Emit(OpCodes.Call, typeof(CData).GetProperty(nameof(CData.UnsafeAddress))!.GetGetMethod()!);
                 method.Emit(OpCodes.Ldobj, ((INativeType)this).GetNativeType());
                 return null;
             }
@@ -165,19 +162,19 @@ string INativeType.TypeFormat {
 
             #endregion
 
-            private void SetFields(object fields) {
+            [MemberNotNull(nameof(_fields))]
+            private void SetFields(object? fields) {
                 lock (this) {
-                    IList<object> list = GetFieldsList(fields);
-                    IList<object> anonFields = StructType.GetAnonymousFields(this);
+                    IList<object> fieldDefList = GetFieldsList(fields);
+                    IList<object>? anonFields = StructType.GetAnonymousFields(this);
 
                     int size = 0, alignment = 1;
                     List<Field> allFields = new List<Field>();//GetBaseSizeAlignmentAndFields(out size, out alignment);
                     int? bitCount;
-                    for (int fieldIndex = 0; fieldIndex < list.Count; fieldIndex++) {
-                        object o = list[fieldIndex];
+                    foreach (object fieldDef in fieldDefList) {
                         string fieldName;
                         INativeType cdata;
-                        GetFieldInfo(this, o, out fieldName, out cdata, out bitCount);
+                        GetFieldInfo(this, fieldDef, out fieldName, out cdata, out bitCount);
                         alignment = Math.Max(alignment, cdata.Alignment);
                         size = Math.Max(size, cdata.Size);
 
@@ -192,12 +189,13 @@ private void SetFields(object fields) {
 
                     StructType.CheckAnonymousFields(allFields, anonFields);
 
-                    _fields = allFields.ToArray();
+                    _fields = [..allFields];
                     _size = PythonStruct.Align(size, alignment);
                     _alignment = alignment;
                 }
             }
 
+            [MemberNotNull(nameof(_fields))]
             internal void EnsureFinal() {
                 if (_fields == null) {
                     SetFields(PythonTuple.EMPTY);
diff --git a/src/core/IronPython.Modules/_ctypes/_ctypes.cs b/src/core/IronPython.Modules/_ctypes/_ctypes.cs
index acdd300c6..44a554de9 100644
--- a/src/core/IronPython.Modules/_ctypes/_ctypes.cs
+++ b/src/core/IronPython.Modules/_ctypes/_ctypes.cs
@@ -633,8 +633,8 @@ private static int CheckBits(INativeType cdata, PythonTuple pt) {
         /// Shared helper to get the _fields_ list for struct/union and validate it.
         /// </summary>
         private static IList<object>/*!*/ GetFieldsList(object fields) {
-            if (!(fields is IList<object> list)) {
-                throw PythonOps.TypeError("class must be a sequence of pairs");
+            if (fields is not IList<object> list) {
+                throw PythonOps.TypeError("'_fields_' must be a sequence of (name, C type) pairs");
             }
             return list;
         }