diff --git a/rest_witchcraft/serializers.py b/rest_witchcraft/serializers.py
index 97616c1..e15f300 100644
--- a/rest_witchcraft/serializers.py
+++ b/rest_witchcraft/serializers.py
@@ -42,7 +42,7 @@ def is_nested(self):
 
     def build_standard_field_kwargs(self, field_name, field_class, column_info):
         """Analyze model column to generate field kwargs."""
-        field_kwargs = column_info.field_kwargs.copy()
+        field_kwargs = copy.deepcopy(column_info.field_kwargs)
         field_kwargs["label"] = capfirst(" ".join(field_name.split("_")).strip())
         field_kwargs["allow_null"] = not field_kwargs.get("required", True)
 
@@ -383,13 +383,16 @@ def get_field_names(self, declared_fields, info):
             serializer_class=self.__class__.__name__
         )
 
-        assert not (_fields is None and exclude is None), (
+        assert _fields is not None or exclude is not None, (
             "Creating a ModelSerializer without either the 'fields' attribute "
             "or the 'exclude' attribute has been deprecated since 3.3.0, "
             "and is now disallowed. Add an explicit fields = '__all__' to the "
-            "{serializer_class} serializer.".format(serializer_class=self.__class__.__name__),
+            "{serializer_class} serializer.".format(
+                serializer_class=self.__class__.__name__
+            ),
         )
 
+
         if _fields == ALL_FIELDS:
             _fields = None
 
@@ -574,10 +577,11 @@ def get_nested_relationship_fields(self, relation_info, depth):
         target_model_info = meta.model_info(relation_info.related_model)
 
         # figure out backrefs
-        backrefs = set()
-        for key, rel in target_model_info.relationships.items():
-            if rel.related_model == self.model:
-                backrefs.add(key)
+        backrefs = {
+            key
+            for key, rel in target_model_info.relationships.items()
+            if rel.related_model == self.model
+        }
 
         _fields = set(target_model_info.primary_keys.keys())
         _fields.update(target_model_info.properties.keys())