New API code for unknown UUIDs on lookup?

[CrsiX]

POST /api/v2/friends returns for unknown UUIDs:

{
  "message": "Invalid username",
  "status_code": 1009
}

Change to "unknown UUID" or introduce a new field (aka more ApiErrorResponse types) for that? After all, the code 1009 will be mapped to something like "the username is invalid, as it may contain forbidden characters".