diff --git a/tests/rest/client/v2_alpha/test_register.py b/tests/rest/client/v2_alpha/test_register.py
index b6173ab2ee..821c735528 100644
--- a/tests/rest/client/v2_alpha/test_register.py
+++ b/tests/rest/client/v2_alpha/test_register.py
@@ -47,6 +47,7 @@ class RegisterRestServletTestCase(unittest.TestCase):
         self.hs.get_auth_handler = Mock(return_value=self.auth_handler)
         self.hs.get_device_handler = Mock(return_value=self.device_handler)
         self.hs.config.enable_registration = True
+        self.hs.config.auto_join_rooms = []
 
         # init the thing we're testing
         self.servlet = RegisterRestServlet(self.hs)