diff --git a/stdlib/3/types.pyi b/stdlib/3/types.pyi
index b8f7d925a..f826ce91d 100644
--- a/stdlib/3/types.pyi
+++ b/stdlib/3/types.pyi
@@ -248,7 +248,7 @@ class TracebackType:
     def tb_lineno(self) -> int: ...
 
 class FrameType:
-    f_back: FrameType
+    f_back: Optional[FrameType]
     f_builtins: Dict[str, Any]
     f_code: CodeType
     f_globals: Dict[str, Any]