diff --git a/django/utils/datastructures.py b/django/utils/datastructures.py
index a15f09ca7b..3d7c4275bb 100644
--- a/django/utils/datastructures.py
+++ b/django/utils/datastructures.py
@@ -124,8 +124,9 @@ class MultiValueDict(dict):
     def __copy__(self):
         return self.__class__(dict.items(self))
 
-    def __deepcopy__(self, memo):
+    def __deepcopy__(self, memo=None):
         import copy
+        if memo is None: memo = {}
         result = self.__class__()
         memo[id(self)] = result
         for key, value in dict.items(self):