diff --git a/gerrit_util.py b/gerrit_util.py
index 1923b6ad5..baec860c5 100644
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -140,6 +140,10 @@ class CookiesAuthenticator(Authenticator):
 
   @classmethod
   def get_new_password_message(cls, host):
+    if host is None:
+      return ('Git host for gerrit upload is unknown. Check your remote '
+              'and the branch your branch is tracking. This tool assumes '
+              'that you are using a git server at *.googlesource.com.')
     assert not host.startswith('http')
     # Assume *.googlesource.com pattern.
     parts = host.split('.')