Update Contact via API not updating custom field

contacts

#1

Hi I am attempting to update a custom field via an API call. I receive the 204 status code but when I look at my contact on the webapp or by using the API, no data has been modified.

https://api.hubapi.com/contacts/v1/contact/vid/151/profile?hapikey=secretstuff
{"properties": [
    { "property": "idm_id", "value": "1234-abcd-567-efg"}
  ] }

I then receive a 204 No Content status code.
However, the field is still blank and shows no sign of any update made.