diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index 07e28c295..58385c58c 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -627,7 +627,7 @@ class Interpreter(InterpreterBase, HoldableObject):
             ext_module = NotFoundExtensionModule(real_modname)
         else:
             ext_module = module.initialize(self)
-            assert isinstance(ext_module, (ExtensionModule, NewExtensionModule))
+            assert isinstance(ext_module, (ExtensionModule, NewExtensionModule)), 'for mypy'
             self.build.modules.add(real_modname)
         if ext_module.INFO.added:
             FeatureNew.single_use(f'module {ext_module.INFO.name}', ext_module.INFO.added, self.subproject, location=node)