diff --git a/stdlib/3/collections/__init__.pyi b/stdlib/3/collections/__init__.pyi
index 52470a3bf567..600e3d5001a8 100644
--- a/stdlib/3/collections/__init__.pyi
+++ b/stdlib/3/collections/__init__.pyi
@@ -321,3 +321,4 @@ class ChainMap(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     def __getitem__(self, k: _KT) -> _VT: ...
     def __iter__(self) -> Iterator[_KT]: ...
     def __len__(self) -> int: ...
+    def __missing__(self, key: _KT) -> _VT: ...  # undocumented