diff --git a/googleapiclient/discovery.py b/googleapiclient/discovery.py
index 441dbf0..f3e5690 100644
--- a/googleapiclient/discovery.py
+++ b/googleapiclient/discovery.py
@@ -712,7 +712,7 @@
         raise TypeError('media_filename must be str or MediaUpload.')
 
       # Check the maxSize
-      if maxSize > 0 and media_upload.size() > maxSize:
+      if media_upload.size() is not None and media_upload.size() > maxSize > 0:
         raise MediaUploadSizeError("Media larger than: %s" % maxSize)
 
       # Use the media path uri for media uploads