diff --git a/googleapiclient/http.py b/googleapiclient/http.py
index f5d08a19a01..6aac030b8a7 100644
--- a/googleapiclient/http.py
+++ b/googleapiclient/http.py
@@ -1207,7 +1207,7 @@ def _serialize_request(self, request):
 
     if request.http is not None:
       credentials = _auth.get_credentials_from_http(request.http)
-      if credentials is not None:
+      if credentials is not None and getattr(credentials, 'access_token', None):
         _auth.apply_credentials(credentials, headers)
 
     # MIMENonMultipart adds its own Content-Type header.