diff --git a/pymongo/write_concern.py b/pymongo/write_concern.py
index d62c3c311..6487197b0 100644
--- a/pymongo/write_concern.py
+++ b/pymongo/write_concern.py
@@ -118,7 +118,7 @@ class WriteConcern:
         """
         return self.__acknowledged
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return "WriteConcern({})".format(
             ", ".join("{}={}".format(*kvt) for kvt in self.__document.items())
         )