diff --git a/DMCompiler/Compiler/DM/AST/DMAST.ObjectStatements.cs b/DMCompiler/Compiler/DM/AST/DMAST.ObjectStatements.cs
index a2f210bdfc..e0744b4f5d 100644
--- a/DMCompiler/Compiler/DM/AST/DMAST.ObjectStatements.cs
+++ b/DMCompiler/Compiler/DM/AST/DMAST.ObjectStatements.cs
@@ -59,7 +59,7 @@ public DMASTProcDefinition(Location location, DreamPath path, DMASTDefinitionPar
         }
 
         ObjectPath = (path.Elements.Length > 1) ? path.FromElements(0, -2) : DreamPath.Root;
-        Name = path.LastElement;
+        Name = path.LastElement ?? throw new ArgumentException($"Proc path \"{path}\" is missing a name", nameof(path));
         Parameters = parameters;
         Body = body;
         ReturnTypes = returnType;
@@ -103,7 +103,7 @@ public sealed class DMASTObjectVarOverride : DMASTStatement {
 
     public DMASTObjectVarOverride(Location location, DreamPath path, DMASTExpression value) : base(location) {
         ObjectPath = path.FromElements(0, -2);
-        VarName = path.LastElement;
+        VarName = path.LastElement ?? throw new ArgumentException($"Var override path \"{path}\" is missing a name", nameof(path));
         Value = value;
     }
 }
diff --git a/DMCompiler/Compiler/DM/DMParser.cs b/DMCompiler/Compiler/DM/DMParser.cs
index 0996f41bf0..543132d395 100644
--- a/DMCompiler/Compiler/DM/DMParser.cs
+++ b/DMCompiler/Compiler/DM/DMParser.cs
@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using DMCompiler.Compiler.DMPreprocessor;
 using System.Linq;
 using DMCompiler.Compiler.DM.AST;
@@ -506,6 +507,7 @@ public DMASTFile File() {
             return null;
         }
 
+        [return: NotNullIfNotNull(nameof(expression))]
         private DMASTExpression? ParseScopeIdentifier(DMASTExpression? expression) {
             do {
                 var identifier = Identifier();
@@ -1265,6 +1267,7 @@ private DMASTProcStatement For() {
             if (Check(TokenType.DM_In)) {
                 Whitespace();
                 DMASTExpression? listExpr = Expression();
+                RequireExpression(ref listExpr);
                 Whitespace();
                 Consume(TokenType.DM_RightParenthesis, "Expected ')' in for after expression 2");
                 ExtraColonPeriod();
diff --git a/DMCompiler/Compiler/DMPreprocessor/DMPreprocessor.cs b/DMCompiler/Compiler/DMPreprocessor/DMPreprocessor.cs
index 8ed167d0ab..007370f04d 100644
--- a/DMCompiler/Compiler/DMPreprocessor/DMPreprocessor.cs
+++ b/DMCompiler/Compiler/DMPreprocessor/DMPreprocessor.cs
@@ -197,8 +197,8 @@ public void DefineMacro(string key, string value) {
 
     // NB: Pushes files to a stack, so call in reverse order if you are
     // including multiple files.
-    public void IncludeFile(string includeDir, string file, bool isDMStandard, Location? includedFrom = null) {
-        string filePath = Path.Combine(includeDir, file);
+    public void IncludeFile(string? includeDir, string file, bool isDMStandard, Location? includedFrom = null) {
+        string filePath = Path.Combine(includeDir ?? string.Empty, file);
         filePath = filePath.Replace('\\', Path.DirectorySeparatorChar);
         filePath = Path.GetFullPath(filePath); // Strips out path operators
 
@@ -243,7 +243,7 @@ public void IncludeFile(string includeDir, string file, bool isDMStandard, Locat
         }
     }
 
-    public void PreprocessFile(string includeDir, string file, bool isDMStandard) {
+    public void PreprocessFile(string? includeDir, string file, bool isDMStandard) {
         file = file.Replace('\\', '/');
 
         _lexerStack.Push(new DMPreprocessorLexer(compiler, includeDir, file, isDMStandard));
@@ -275,7 +275,7 @@ private void HandleIncludeDirective(Token includeToken) {
 
         DMPreprocessorLexer currentLexer = _lexerStack.Peek();
         string file = Path.Combine(Path.GetDirectoryName(currentLexer.File.Replace('\\', Path.DirectorySeparatorChar)), includedFileToken.ValueAsString());
-        string directory = currentLexer.IncludeDirectory;
+        string? directory = currentLexer.IncludeDirectory;
 
         IncludeFile(directory, file, includeToken.Location.InDMStandard, includedFrom: includeToken.Location);
     }
@@ -307,7 +307,7 @@ private void HandleDefineDirective(Token defineToken) {
             }
 
             DMPreprocessorLexer currentLexer = _lexerStack.Peek();
-            string dir = Path.Combine(currentLexer.IncludeDirectory, dirTokenValue);
+            string dir = Path.Combine(currentLexer.IncludeDirectory ?? string.Empty, dirTokenValue);
             compiler.AddResourceDirectory(dir, dirToken.Location);
 
             // In BYOND it goes on to set the FILE_DIR macro's value to the added directory
diff --git a/DMCompiler/Compiler/DMPreprocessor/DMPreprocessorLexer.cs b/DMCompiler/Compiler/DMPreprocessor/DMPreprocessorLexer.cs
index 3d02133665..1eb8fde8a5 100644
--- a/DMCompiler/Compiler/DMPreprocessor/DMPreprocessorLexer.cs
+++ b/DMCompiler/Compiler/DMPreprocessor/DMPreprocessorLexer.cs
@@ -12,7 +12,7 @@ namespace DMCompiler.Compiler.DMPreprocessor;
 internal sealed class DMPreprocessorLexer {
     private static readonly StringBuilder TokenTextBuilder = new();
 
-    public readonly string IncludeDirectory;
+    public readonly string? IncludeDirectory;
     public readonly string File;
 
     private readonly DMCompiler _compiler;
@@ -23,7 +23,7 @@ internal sealed class DMPreprocessorLexer {
     private int _previousLine = 1, _previousColumn;
     private readonly Queue<Token> _pendingTokenQueue = new(); // TODO: Possible to remove this?
 
-    public DMPreprocessorLexer(DMCompiler compiler, string includeDirectory, string file, string source) {
+    public DMPreprocessorLexer(DMCompiler compiler, string? includeDirectory, string file, string source) {
         _compiler = compiler;
         IncludeDirectory = includeDirectory;
         File = file;
@@ -32,12 +32,12 @@ public DMPreprocessorLexer(DMCompiler compiler, string includeDirectory, string
         Advance();
     }
 
-    public DMPreprocessorLexer(DMCompiler compiler, string includeDirectory, string file, bool isDMStandard) {
+    public DMPreprocessorLexer(DMCompiler compiler, string? includeDirectory, string file, bool isDMStandard) {
         _compiler = compiler;
         IncludeDirectory = includeDirectory;
         File = file;
 
-        _source = new StreamReader(Path.Combine(includeDirectory, file), Encoding.UTF8);
+        _source = new StreamReader(Path.Combine(includeDirectory ?? string.Empty, file), Encoding.UTF8);
         _isDMStandard = isDMStandard;
         Advance();
     }
diff --git a/DMCompiler/DM/Builders/DMASTFolder.cs b/DMCompiler/DM/Builders/DMASTFolder.cs
index 0bbe08b1ea..12291946cc 100644
--- a/DMCompiler/DM/Builders/DMASTFolder.cs
+++ b/DMCompiler/DM/Builders/DMASTFolder.cs
@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using DMCompiler.Compiler.DM.AST;
 
 namespace DMCompiler.DM.Builders;
@@ -122,7 +123,8 @@ public void FoldAst(DMASTNode? ast) {
         }
     }
 
-    private DMASTExpression FoldExpression(DMASTExpression? expression) {
+    [return: NotNullIfNotNull(nameof(expression))]
+    private DMASTExpression? FoldExpression(DMASTExpression? expression) {
         if (expression is DMASTUnary unary) {
             unary.Value = FoldExpression(unary.Value);
         } else if (expression is DMASTBinary binary) {
diff --git a/DMCompiler/DM/Builders/DMProcBuilder.cs b/DMCompiler/DM/Builders/DMProcBuilder.cs
index e93d90a0d8..d1564ccda7 100644
--- a/DMCompiler/DM/Builders/DMProcBuilder.cs
+++ b/DMCompiler/DM/Builders/DMProcBuilder.cs
@@ -19,7 +19,7 @@ public void ProcessProcDefinition(DMASTProcDefinition procDefinition) {
 
             if (parameter.Value != null) { //Parameter has a default value
                 string afterDefaultValueCheck = proc.NewLabelName();
-                DMReference parameterRef = proc.GetLocalVariableReference(parameterName);
+                DMReference parameterRef = proc.GetLocalVariableReference(parameterName, parameter.Location);
 
                 //Don't set parameter to default if not null
                 proc.PushReferenceValue(parameterRef);
@@ -197,7 +197,7 @@ private void ProcessStatementVarDeclaration(DMASTProcStatementVarDeclaration var
         }
 
         value.EmitPushValue(ExprContext);
-        proc.Assign(proc.GetLocalVariableReference(varDeclaration.Name));
+        proc.Assign(proc.GetLocalVariableReference(varDeclaration.Name, varDeclaration.Location));
         proc.Pop();
     }
 
@@ -396,10 +396,11 @@ private void ProcessStatementFor(DMASTProcStatementFor statementFor) {
         }
         proc.EndScope();
 
-        IEnumerable<DMASTVarDeclExpression> FindVarDecls(DMASTExpression expr) {
-            if (expr is DMASTVarDeclExpression p) {
+        IEnumerable<DMASTVarDeclExpression> FindVarDecls(DMASTExpression? expr) {
+            if (expr is null)
+                yield break;
+            if (expr is DMASTVarDeclExpression p)
                 yield return p;
-            }
 
             foreach (var leaf in expr.Leaves()) {
                 foreach(var decl in FindVarDecls(leaf)) {
@@ -863,7 +864,7 @@ private void ProcessStatementTryCatch(DMASTProcStatementTryCatch tryCatch) {
                 compiler.Emit(WarningCode.DuplicateVariable, param.Location, $"Duplicate var {param.Name}");
             }
 
-            proc.StartTry(catchLabel, proc.GetLocalVariableReference(param.Name));
+            proc.StartTry(catchLabel, proc.GetLocalVariableReference(param.Name, param.Location));
         } else {
             if (tryCatch.CatchParameter != null)
                 compiler.Emit(WarningCode.InvalidVarDefinition, tryCatch.CatchParameter.Location,
diff --git a/DMCompiler/DM/DMProc.cs b/DMCompiler/DM/DMProc.cs
index df36b4afeb..3f9e60b83f 100644
--- a/DMCompiler/DM/DMProc.cs
+++ b/DMCompiler/DM/DMProc.cs
@@ -532,9 +532,14 @@ public bool TryAddLocalConstVariable(string name, DreamPath? type, Constant valu
         return null;
     }
 
-    public DMReference GetLocalVariableReference(string name) {
+    public DMReference GetLocalVariableReference(string name, Location loc) {
         LocalVariable? local = GetLocalVariable(name);
 
+        if (local is null) {
+            _compiler.Emit(WarningCode.InvalidReference, loc, $"Attempted to reference invalid local var \"{name}\"");
+            return DMReference.Invalid;
+        }
+
         return local.IsParameter ? DMReference.CreateArgument(local.Id) : DMReference.CreateLocal(local.Id);
     }
 
diff --git a/DMCompiler/DMCompiler.cs b/DMCompiler/DMCompiler.cs
index f55c5e55bc..f0afab4843 100644
--- a/DMCompiler/DMCompiler.cs
+++ b/DMCompiler/DMCompiler.cs
@@ -127,11 +127,12 @@ public void AddResourceDirectory(string dir, Location loc) {
                     return null;
                 }
 
-                string includeDir = Path.GetDirectoryName(files[i]);
+                string? includeDir = Path.GetDirectoryName(files[i]);
                 string fileName = Path.GetFileName(files[i]);
 
                 preproc.IncludeFile(includeDir, fileName, false);
-                compiler.AddResourceDirectory(includeDir, Location.Internal);
+                if (includeDir is not null)
+                    compiler.AddResourceDirectory(includeDir, Location.Internal);
             }
 
             // Adds the root of the DM project to FILE_DIR
@@ -160,8 +161,8 @@ public void AddResourceDirectory(string dir, Location loc) {
                     result.Append(t.Text);
                 }
 
-                string outputDir = Path.GetDirectoryName(Settings.Files[0]);
-                string outputPath = Path.Combine(outputDir, "preprocessor_dump.dm");
+                string? outputDir = Path.GetDirectoryName(Settings.Files[0]);
+                string outputPath = Path.Combine(outputDir ?? string.Empty, "preprocessor_dump.dm");
 
                 File.WriteAllText(outputPath, result.ToString());
                 Console.WriteLine($"Preprocessor output dumped to {outputPath}");