diff --git a/synapse/rest/client/v1/register.py b/synapse/rest/client/v1/register.py
index a7c9c5bb6..86519fd9d 100644
--- a/synapse/rest/client/v1/register.py
+++ b/synapse/rest/client/v1/register.py
@@ -354,7 +354,7 @@ class RegisterRestServlet(ClientV1RestServlet):
             })
         else:
             raise SynapseError(
-                400, "HMAC incorrect",
+                403, "HMAC incorrect",
             )