diff --git a/django/contrib/admin/sites.py b/django/contrib/admin/sites.py
index abcff14cd1..5f397ecb01 100644
--- a/django/contrib/admin/sites.py
+++ b/django/contrib/admin/sites.py
@@ -300,7 +300,7 @@ class AdminSite(object):
         user = authenticate(username=username, password=password)
         if user is None:
             message = ERROR_MESSAGE
-            if u'@' in username:
+            if username is not None and u'@' in username:
                 # Mistakenly entered e-mail address instead of username? Look it up.
                 try:
                     user = User.objects.get(email=username)
diff --git a/tests/regressiontests/admin_views/tests.py b/tests/regressiontests/admin_views/tests.py
index aafa303cec..669755051e 100644
--- a/tests/regressiontests/admin_views/tests.py
+++ b/tests/regressiontests/admin_views/tests.py
@@ -353,6 +353,9 @@ class AdminViewPermissionsTest(TestCase):
                      LOGIN_FORM_KEY: 1,
                      'username': 'joepublic',
                      'password': 'secret'}
+        self.no_username_login = {
+                     LOGIN_FORM_KEY: 1,
+                     'password': 'secret'}
 
     def testLogin(self):
         """
@@ -416,6 +419,14 @@ class AdminViewPermissionsTest(TestCase):
         # Login.context is a list of context dicts we just need to check the first one.
         self.assert_(login.context[0].get('error_message'))
 
+        # Requests without username should not return 500 errors.
+        request = self.client.get('/test_admin/admin/')
+        self.failUnlessEqual(request.status_code, 200)
+        login = self.client.post('/test_admin/admin/', self.no_username_login)
+        self.failUnlessEqual(login.status_code, 200)
+        # Login.context is a list of context dicts we just need to check the first one.
+        self.assert_(login.context[0].get('error_message'))
+
     def testLoginSuccessfullyRedirectsToOriginalUrl(self):
         request = self.client.get('/test_admin/admin/')
         self.failUnlessEqual(request.status_code, 200)