diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 63fab3dc3..473a770aa 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -448,7 +448,10 @@ class Serializer(BaseSerializer, metaclass=SerializerMetaclass):
                 default = field.get_default()
             except SkipField:
                 continue
-            defaults[field.source] = default
+            if field.source == None:
+                defaults[field.field_name] = default
+            else:
+                defaults[field.source] = default
 
         return defaults