diff --git a/zerver/forms.py b/zerver/forms.py
index afc4e6901e..201c5d0c0c 100644
--- a/zerver/forms.py
+++ b/zerver/forms.py
@@ -99,9 +99,11 @@ class RegistrationForm(forms.Form):
 
     def clean_realm_subdomain(self):
         # type: () -> str
+        if not self.realm_creation:
+            return Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
         subdomain = self.cleaned_data['realm_subdomain']
         if not subdomain:
-            return ''
+            return Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
         check_subdomain_available(subdomain)
         return subdomain
 
diff --git a/zerver/tests/test_signup.py b/zerver/tests/test_signup.py
index d14307edda..2e1d070153 100644
--- a/zerver/tests/test_signup.py
+++ b/zerver/tests/test_signup.py
@@ -305,7 +305,7 @@ class LoginTest(ZulipTestCase):
         with queries_captured() as queries:
             self.register(self.nonreg_email('test'), "test")
         # Ensure the number of queries we make is not O(streams)
-        self.assert_length(queries, 69)
+        self.assert_length(queries, 68)
         user_profile = self.nonreg_user('test')
         self.assertEqual(get_session_dict_user(self.client.session), user_profile.id)
         self.assertFalse(user_profile.enable_stream_desktop_notifications)