M_INVALID_PARAM "Invalid login submission" error when trying to use m.login.registration_token

[Half-Shot]

I'm sending a request via curl:

curl --data '{}' 'http://localhost:8008/_matrix/client/v3/register'
...
curl --data '{"password": "password", "auth": { "type": "m.login.registration_token", "token": "hBYSAPs-RSr7ITNk", "session": "nZHgKJUTRaiyPojuVKQUOWhB" }}' 'http://localhost:8008/_matrix/client/v3/register'

Which causes an error of M_INVALID_PARAM.

which is only raised in

synapse/synapse/handlers/auth.py

Lines 124 to 125 in bfe6d55

	if not identifier:
	raise SynapseError(400, "Invalid login submission", Codes.INVALID_PARAM)

.

I have also tried including a username, but that also gives the same error, as does providing a identifier key.

According to the spec, the key isn't even mandatory:

The basis for the localpart of the desired Matrix ID. If omitted, the homeserver MUST generate a Matrix ID local part.

This is on Synapse 1.52.0

[Half-Shot]

This was probably due to #11949, it's a bit of a shame we don't immediately error on a bad auth.type.