diff --git a/tests/rest/utils.py b/tests/rest/utils.py
index ce2e8fd98a..25ed1388cf 100644
--- a/tests/rest/utils.py
+++ b/tests/rest/utils.py
@@ -95,8 +95,14 @@ class RestTestCase(unittest.TestCase):
 
     @defer.inlineCallbacks
     def register(self, user_id):
-        (code, response) = yield self.mock_resource.trigger("POST", "/register",
-                                '{"user_id":"%s"}' % user_id)
+        (code, response) = yield self.mock_resource.trigger(
+            "POST",
+            "/register",
+            json.dumps({
+                "user_id": user_id,
+                "password": "test",
+                "type": "m.login.password"
+            }))
         self.assertEquals(200, code)
         defer.returnValue(response)