diff --git a/gridfs/grid_file.py b/gridfs/grid_file.py
index 463213825..2185b3213 100644
--- a/gridfs/grid_file.py
+++ b/gridfs/grid_file.py
@@ -297,7 +297,7 @@ class GridIn(object):
             raise ValueError("cannot write to a closed file")
 
         # With w=0, 'filemd5' might run before the final chunks are written.
-        if 0 == self._coll.database.connection.write_concern.get('w'):
+        if not self._coll.acknowledged:
             raise InvalidOperation('Cannot write file to GridFS with w=0')
 
         try: