diff --git a/mypy/build.py b/mypy/build.py
index 06d5ac3f72cf..e76ca2b1119a 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -494,6 +494,7 @@ def take_module_snapshot(module: types.ModuleType) -> str:
     (e.g. if there is a change in modules imported by a plugin).
     """
     if hasattr(module, '__file__'):
+        assert module.__file__ is not None
         with open(module.__file__, 'rb') as f:
             digest = hash_digest(f.read())
     else: