1 files changed, 3 insertions, 1 deletions
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index 999bc6efb5..ccd96e4626 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -124,7 +124,9 @@ class RegistrationHandler(BaseHandler):
             try:
                 int(localpart)
                 raise SynapseError(
-                    400, "Numeric user IDs are reserved for guest users."
+                    400,
+                    "Numeric user IDs are reserved for guest users.",
+                    errcode=Codes.INVALID_USERNAME,
                 )
             except ValueError:
                 pass