Problem setting named_users on development

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I am having an issue with calling the named_user/associate API on my development app.

I am POST'ing the data as for example:

{
  "channel_id": "cd0ba892-a584-4d08-94a0-XXXXXXXXX",
  "device_type": "ios",
  "named_user_id": "4681649"
}

but it doesn't seem to associate the channel_id properly. When I query for that named_user_id it returns an empty channel list:

"named_user": {
  "named_user_id": "4681649",
  "tags": {},
  "created": "2018-09-05T16:58:00",
  "last_modified": "2018-09-05T17:14:28",
  "channels": []
}