diff --git a/stdlib/3/collections/__init__.pyi b/stdlib/3/collections/__init__.pyi
index 88c2c95aa..1cde078da 100644
--- a/stdlib/3/collections/__init__.pyi
+++ b/stdlib/3/collections/__init__.pyi
@@ -62,6 +62,7 @@ else:
 _UserDictT = TypeVar('_UserDictT', bound=UserDict)
 
 class UserDict(MutableMapping[_KT, _VT]):
+    data = ...  # type: Mapping[_KT, _VT]
     def __len__(self) -> int: ...
     def __getitem__(self, key: _KT) -> _VT: ...
     def __setitem__(self, key: _KT, item: _VT) -> None: ...