diff --git a/mypy/main.py b/mypy/main.py
index 0c2cb6d6090d2..b04fa2a8db503 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -84,6 +84,10 @@ def flush_errors(new_messages: List[str], serious: bool) -> None:
         except BrokenPipeError:
             sys.exit(2)
 
+    if options.python_version >= (3, 9):
+        from mypy.nodes import nongen_builtins
+        nongen_builtins.clear()
+
     serious = False
     blockers = False
     res = None