diff --git a/changelog.d/8135.bugfix b/changelog.d/8135.bugfix
new file mode 100644
index 0000000000..9d5c60ea00
--- /dev/null
+++ b/changelog.d/8135.bugfix
@@ -0,0 +1 @@
+Clarify the error code if a user tries to register with a numeric ID. This bug was introduced in v1.15.0.
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