type: 'https://mailchimp.com/developer/marketing/docs/errors/',

title: 'Invalid Resource',

status: 400,

detail: "The resource submitted could not be validated. For field-specific details, see the 'errors' array.",

instance: '4711820f-f2e2-6452-5060-40fa35ed3f89',

errors: [ { field: '', message: 'Schema describes object, NULL found instead'

Even on the local system, it isn't working. Tried merge_fields entry and all the debugging with jsonlinter, but nothing worked for me.