diff --git a/derive/src/pyclass.rs b/derive/src/pyclass.rs
index 606cd6b27..0ed7081b4 100644
--- a/derive/src/pyclass.rs
+++ b/derive/src/pyclass.rs
@@ -295,6 +295,12 @@ fn extract_impl_items(mut items: Vec<ItemIdent>) -> Result<TokenStream2, Diagnos
 fn extract_impl_attrs(attr: AttributeArgs) -> Result<(TokenStream2, TokenStream2), Diagnostic> {
     let mut withs = Vec::new();
     let mut flags = vec![quote! { ::rustpython_vm::slots::PyTpFlags::DEFAULT.bits() }];
+    #[cfg(debug_assertions)]
+    {
+        flags.push(quote! {
+            | ::rustpython_vm::slots::PyTpFlags::_CREATED_WITH_FLAGS.bits()
+        });
+    }
 
     for attr in attr {
         match attr {
diff --git a/vm/src/obj/objmodule.rs b/vm/src/obj/objmodule.rs
index 5a000e661..2cf988311 100644
--- a/vm/src/obj/objmodule.rs
+++ b/vm/src/obj/objmodule.rs
@@ -55,6 +55,9 @@ impl PyModuleRef {
         doc: OptionalOption<PyStringRef>,
         vm: &VirtualMachine,
     ) -> PyResult<()> {
+        debug_assert!(crate::pyobject::TypeProtocol::lease_class(self.as_object())
+            .flags
+            .has_feature(crate::slots::PyTpFlags::HAS_DICT));
         init_module_dict(
             vm,
             &self.as_object().dict().unwrap(),
diff --git a/vm/src/pyobject.rs b/vm/src/pyobject.rs
index d9b69a601..53e5f7a89 100644
--- a/vm/src/pyobject.rs
+++ b/vm/src/pyobject.rs
@@ -1446,6 +1446,7 @@ pub trait PyClassImpl: PyClassDef {
     fn impl_extend_class(ctx: &PyContext, class: &PyClassRef);
 
     fn extend_class(ctx: &PyContext, class: &PyClassRef) {
+        debug_assert!(class.flags.is_created_with_flags());
         if Self::TP_FLAGS.has_feature(PyTpFlags::HAS_DICT) {
             class.set_str_attr(
                 "__dict__",
@@ -1472,6 +1473,10 @@ pub trait PyClassImpl: PyClassDef {
         Self::extend_class(ctx, &py_class);
         py_class
     }
+
+    fn create_bare_type(type_type: &PyClassRef, base: &PyClassRef) -> PyClassRef {
+        create_type_with_flags(Self::NAME, type_type, base, Self::TP_FLAGS)
+    }
 }
 
 pub trait PyStructSequenceImpl: PyClassImpl {
diff --git a/vm/src/slots.rs b/vm/src/slots.rs
index 586f6c2e6..adecc7067 100644
--- a/vm/src/slots.rs
+++ b/vm/src/slots.rs
@@ -7,6 +7,9 @@ bitflags! {
         const HEAPTYPE = 1 << 9;
         const BASETYPE = 1 << 10;
         const HAS_DICT = 1 << 40;
+
+        #[cfg(debug_assertions)]
+        const _CREATED_WITH_FLAGS = 1 << 63;
     }
 }
 
@@ -17,6 +20,14 @@ impl PyTpFlags {
     pub fn has_feature(self, flag: Self) -> bool {
         self.contains(flag)
     }
+
+    pub fn is_created_with_flags(self) -> bool {
+        if cfg!(debug_assertions) {
+            self.contains(Self::_CREATED_WITH_FLAGS)
+        } else {
+            true
+        }
+    }
 }
 
 impl Default for PyTpFlags {