diff --git a/mypyc/irbuild/classdef.py b/mypyc/irbuild/classdef.py
index 84dd493c6d15..dda8f31fd893 100644
--- a/mypyc/irbuild/classdef.py
+++ b/mypyc/irbuild/classdef.py
@@ -97,6 +97,10 @@ def transform_class_def(builder: IRBuilder, cdef: ClassDef) -> None:
 
     This is the main entry point to this module.
     """
+    if cdef.info not in builder.mapper.type_to_ir:
+        builder.error("Nested class definitions not supported", cdef.line)
+        return
+
     ir = builder.mapper.type_to_ir[cdef.info]
 
     # We do this check here because the base field of parent
diff --git a/mypyc/test-data/irbuild-classes.test b/mypyc/test-data/irbuild-classes.test
index dbc1f8927669..e0f7dfe6514f 100644
--- a/mypyc/test-data/irbuild-classes.test
+++ b/mypyc/test-data/irbuild-classes.test
@@ -1300,3 +1300,15 @@ class T:
 
 class E(T):
     y: str  # E: Type of "y" is incompatible with definition in trait "T"
+
+
+[case testNestedClasses]
+def outer():
+    class Inner:  # E: Nested class definitions not supported
+        pass
+
+    return Inner
+
+if True:
+    class OtherInner:  # E: Nested class definitions not supported
+        pass