An exception occurred: 400 Client Error: Bad Request

Trying to send out a test fax and receiving this error.

An exception occurred: 400 Client Error: Bad Request for url: https://rest.interfax.net/outbound/faxes?faxNumber=%2B1xxxxxxxxxx

    interfax = InterFAX(username="username", password="password")
    fax = interfax.outbound.deliver(fax_number="+1xxxxxxxxxx", files=['test.pdf'])
    fax = fax.reload() # resync with API to get latest status
    fax.status # Success if 0. Pending if 0

where the x's represent the fax number