diff --git a/flashcards/fields.py b/flashcards/fields.py
index 016edc7..c3b124e 100644
--- a/flashcards/fields.py
+++ b/flashcards/fields.py
@@ -40,7 +40,7 @@ class MaskField(models.Field):
     def to_python(self, value):
         if value is None:
             return value
-        return sorted(map(list, FlashcardMask(value)))
+        return sorted(list(FlashcardMask(value)))
 
     def get_prep_value(self, value):
         if value is None: