Unable to create ticket due to blank field but it is set

I'm using the /api/v2/requests API to create a new ticket and I'm getting the following error.

Exception: Failed to create support ticket. Status code: 422. Response: {"error":"RecordInvalid","description":"Record validation errors","details":{"base":[{"description":"XXXXXXXXXXX: cannot be blank","error":"BlankValue","field_key":20687399XXXXXX}]}}

I've added the missing field key to both `custom_fields` and the top level `request` of my API payload but I'm still getting this error.