diff --git a/src/websockets/datastructures.py b/src/websockets/datastructures.py
index 37d3b5f86..77b68ba9a 100644
--- a/src/websockets/datastructures.py
+++ b/src/websockets/datastructures.py
@@ -134,7 +134,7 @@ def clear(self) -> None:
         self._dict = {}
         self._list = []
 
-    def update(self, *args: HeadersLike, **kwargs: str) -> None:
+    def update(self, *args: HeadersLike, **kwargs: str) -> None:  # type: ignore
         """
         Update from a :class:`Headers` instance and/or keyword arguments.