ListField(None) raises an exception, so it shouldn't be the default

dictshield/fields/compound.py

@@ -148,7 +148,7 @@ class SortedListField(ListField):
 
     _ordering = None
 
-    def __init__(self, field=None, **kwargs):
+    def __init__(self, field, **kwargs):
         if 'ordering' in kwargs.keys():
             self._ordering = kwargs.pop('ordering')
         super(SortedListField, self).__init__(field, **kwargs)