Internal server error on Create Customer & subscription API

This is the API that we are working with. https://support.billsby.com/reference/#create-a-subscription-for-a-new-customer

When passing all the fields with the card tokenizer I'm getting this error.

&#123;
  "StatusCode": 500,
  "Message": "Internall Server Error",
  "Error": "Value cannot be null. (Parameter \'source\')" 
&#125;

Note: I'm using demo Stripe card 4242424242 for testing.

PS: Also, there is an error in the API documentation, which prevents the Try it button from working. There is an extra space after the company domain in the URL.