diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index d8b82e5fb..a4cc7f3b2 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -181,6 +181,7 @@ class WritableField(Field):
 
         try:
             if self._use_files:
+                files = files or {}  # For the case when files==None
                 native = files[field_name]
             else:
                 native = data[field_name]