diff --git a/Lib/dumbdbm.py b/Lib/dumbdbm.py
index 07fd1b33401..ad3ebcbcb9b 100644
--- a/Lib/dumbdbm.py
+++ b/Lib/dumbdbm.py
@@ -203,6 +203,7 @@ def keys(self):
         return self._index.keys()
 
     def __contains__(self, key):
+        key = key.decode("latin-1")
         return key in self._index
 
     def iterkeys(self):