Getting 400 on an API request constructed from the docs

I’m using the builder here: https://developer.kustomer.com/kustomer-api-docs/reference/createamessagebycustomer

and everything is “green” on my inputs, but I get an error response (below) that the message param is badly formatted. Any ideas?

Here you can see I am inputting a simple string as the message.

here is the error: 

{
  'errors': [
    {
      'status': 400,
      'source': {
        'pointer': '/reference/message'
      },
      'code': 'badparam',
      'title': 'Format validation failed (invalid)'
    }
  ]
}