diff --git a/flashcards/fields.py b/flashcards/fields.py
index 2fe841b..2cd5ad5 100644
--- a/flashcards/fields.py
+++ b/flashcards/fields.py
@@ -45,7 +45,7 @@ class MaskField(models.Field):
     def get_prep_value(self, value):
         if value is None:
             return value
-        return sorted(list(FlashcardMask(value)))
+        return sorted(map(list, FlashcardMask(value)))
 
     def get_prep_lookup(self, lookup_type, value):
         raise TypeError("Lookup not supported for MaskField")