diff --git a/stdlib/3/collections/__init__.pyi b/stdlib/3/collections/__init__.pyi
index 4e460b873..2de35454e 100644
--- a/stdlib/3/collections/__init__.pyi
+++ b/stdlib/3/collections/__init__.pyi
@@ -58,7 +58,6 @@ def namedtuple(typename: str, field_names: Union[str, Iterable[Any]], *,
 class UserDict(MutableMapping): ...
 class UserList(MutableSequence): ...
 class UserString(Sequence): ...
-class MutableString(UserString, MutableSequence): ...
 
 # Technically, deque only derives from MutableSequence in 3.5.
 # But in practice it's not worth losing sleep over.