"incompatible_field" error for a certain ticket type

Hi I am getting this error but I already verified that "Existing Order" type does have the order_id field.  What could be wrong?

{

    "description": "Validation failed",

    "errors": [

        {

            "field": "custom_fields.order_id",

            "message": "It cannot be set, when the type is of value 'Existing Order'",

            "code": "incompatible_field"

        }

    ]

}