diff --git a/oauth2client/appengine.py b/oauth2client/appengine.py
index a9766c7..439a579 100644
--- a/oauth2client/appengine.py
+++ b/oauth2client/appengine.py
@@ -71,7 +71,7 @@
   AssertionFlowCredentials objects may be safely pickled and unpickled.
   """
 
-  def __init__(self, scope, user_agent,
+  def __init__(self, scope,
       audience='https://accounts.google.com/o/oauth2/token',
       assertion_type='http://oauth.net/grant_type/jwt/1.0/bearer',
       token_uri='https://accounts.google.com/o/oauth2/token', **kwargs):
@@ -79,7 +79,6 @@
 
     Args:
       scope: string, scope of the credentials being requested.
-      user_agent: string, The HTTP User-Agent to provide for this application.
       audience: string, The audience, or verifier of the assertion.  For
         convenience defaults to Google's audience.
       assertion_type: string, Type name that will identify the format of the
@@ -94,7 +93,7 @@
 
     super(AppAssertionCredentials, self).__init__(
         assertion_type,
-        user_agent,
+        None,
         token_uri)
 
   def _generate_assertion(self):
@@ -261,8 +260,7 @@
     decorator = OAuth2Decorator(
         client_id='837...ent.com',
         client_secret='Qh...wwI',
-        scope='https://www.googleapis.com/auth/buzz',
-        user_agent='my-sample-app/1.0')
+        scope='https://www.googleapis.com/auth/buzz')
 
 
     class MainHandler(webapp.RequestHandler):
@@ -275,7 +273,7 @@
 
   """
 
-  def __init__(self, client_id, client_secret, scope, user_agent,
+  def __init__(self, client_id, client_secret, scope,
                auth_uri='https://accounts.google.com/o/oauth2/auth',
                token_uri='https://accounts.google.com/o/oauth2/token'):
 
@@ -285,14 +283,13 @@
       client_id: string, client identifier.
       client_secret: string client secret.
       scope: string, scope of the credentials being requested.
-      user_agent: string, HTTP User-Agent to provide for this application.
       auth_uri: string, URI for authorization endpoint. For convenience
         defaults to Google's endpoints but any OAuth 2.0 provider can be used.
       token_uri: string, URI for token endpoint. For convenience
         defaults to Google's endpoints but any OAuth 2.0 provider can be used.
     """
-    self.flow = OAuth2WebServerFlow(client_id, client_secret, scope,
-      user_agent, auth_uri, token_uri)
+    self.flow = OAuth2WebServerFlow(client_id, client_secret, scope, None,
+        auth_uri, token_uri)
     self.credentials = None
     self._request_handler = None
 
diff --git a/oauth2client/client.py b/oauth2client/client.py
index 523a185..f547428 100644
--- a/oauth2client/client.py
+++ b/oauth2client/client.py
@@ -202,9 +202,12 @@
     """Generate the headers that will be used in the refresh request
     """
     headers = {
-        'user-agent': self.user_agent,
         'content-type': 'application/x-www-form-urlencoded',
     }
+
+    if self.user_agent is not None:
+      headers['user-agent'] = self.user_agent
+
     return headers
 
   def _refresh(self, http_request):
@@ -289,10 +292,12 @@
       if headers == None:
         headers = {}
       headers['authorization'] = 'OAuth ' + self.access_token
-      if 'user-agent' in headers:
-        headers['user-agent'] = self.user_agent + ' ' + headers['user-agent']
-      else:
-        headers['user-agent'] = self.user_agent
+
+      if self.user_agent is not None:
+        if 'user-agent' in headers:
+          headers['user-agent'] = self.user_agent + ' ' + headers['user-agent']
+        else:
+          headers['user-agent'] = self.user_agent
 
       resp, content = request_orig(uri, method, body, headers,
                                    redirections, connection_type)
@@ -494,9 +499,12 @@
       'scope': self.scope,
       })
     headers = {
-      'user-agent': self.user_agent,
       'content-type': 'application/x-www-form-urlencoded',
     }
+
+    if self.user_agent is not None:
+      headers['user-agent'] = self.user_agent
+
     if http is None:
       http = httplib2.Http()
     resp, content = http.request(self.token_uri, method='POST', body=body,
diff --git a/samples/appengine_with_robots/main.py b/samples/appengine_with_robots/main.py
index 01ee3a8..5bfe13d 100644
--- a/samples/appengine_with_robots/main.py
+++ b/samples/appengine_with_robots/main.py
@@ -40,8 +40,7 @@
 from oauth2client.appengine import AppAssertionCredentials
 
 credentials = AppAssertionCredentials(
-    scope='https://www.googleapis.com/auth/urlshortener',
-    user_agent='my-sample-app/1.0')
+    scope='https://www.googleapis.com/auth/urlshortener')
 
 http = credentials.authorize(httplib2.Http(memcache))
 service = build("urlshortener", "v1", http=http)
diff --git a/tests/test_oauth2client_appengine.py b/tests/test_oauth2client_appengine.py
index 7f370b3..fd06668 100644
--- a/tests/test_oauth2client_appengine.py
+++ b/tests/test_oauth2client_appengine.py
@@ -93,9 +93,7 @@
                                             app_identity_stub)
 
     self.scope = "http://www.googleapis.com/scope"
-    user_agent = "hal/3.0"
-
-    self.credentials = AppAssertionCredentials(self.scope, user_agent)
+    self.credentials = AppAssertionCredentials(self.scope)
 
   def test_assertion(self):
     assertion = self.credentials._generate_assertion()
@@ -133,8 +131,7 @@
 
     decorator = OAuth2Decorator(client_id='foo_client_id',
                                 client_secret='foo_client_secret',
-                                scope='foo_scope',
-                                user_agent='foo_user_agent')
+                                scope='foo_scope')
     self.decorator = decorator
 
     class TestRequiredHandler(webapp.RequestHandler):