diff --git a/django/db/models/base.py b/django/db/models/base.py
index d847ff7b56..88bed45c8a 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -298,7 +298,7 @@ class Model(object):
         return ''
 
     def _get_FIELD_size(self, field):
-        return os.path.getsize(self.__get_FIELD_filename(field))
+        return os.path.getsize(self._get_FIELD_filename(field))
 
     def _save_FIELD_file(self, field, filename, raw_contents):
         directory = field.get_directory_name()