500 error Marketing Cloud API - Deleting Contacts

Question

Given a successful authorization, the following API call fails with 500

POST

/contacts/v1/contacts/actions/delete?type=keys

Body:

{    
  "DeleteOperationType" : "ContactAndAttributes",
  "Values" : [ "0031Q00001uEjepQAC" ],
  "ContactTypeId" : 0
}

Response

Callout response
Response Status:Internal Server Error
Response Status Code: 500
Response Body: {
  "message" : "Internal Server Error",
  "errorcode" : 0,
  "documentation" : "https://developer.salesforce.com/docs/atlas.en-us.mc-apis.meta/mc-apis/error-handling.htm"
}

Where the documentation link provides this unhelpful suggestion:

Internal Server Error   See the response returned by the HTTP routing assertion.    500

Answer 1

There were layers of custom Apex HTTP callout software involved here and it was the first time I had ever used the SFMC API so this took longer to figure out than it should have (and was a rookie mistake)

You need to be sure that you include this in the header:

Content-Type: application/json

I imagine this solution applies to most if not all of the POST or PATCH API requests. Invalid JSON would be another reason. Doc feedback submitted.