diff --git a/oauth2client/client.py b/oauth2client/client.py
index 1ad94e6..8517946 100644
--- a/oauth2client/client.py
+++ b/oauth2client/client.py
@@ -854,7 +854,7 @@
     body = urllib.urlencode({
         'assertion_type': self.assertion_type,
         'assertion': assertion,
-        'grant_type': 'assertion',
+        'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer',
         })
 
     return body