Problems with POST and forms

I'm trying to use this proxy to forward requests to an ASP.NET server on another domain. When I send a login request (POST to /api/login, request body containing a form) directly to the ASP.NET server it works fine, but when it has passed through the proxy I get an exception:

> System.IO.IOException: Unexpected end of Stream, the content may have already been read by another component.

This exception occurs when I attempt to read the form from the request body.

I'm running the proxy with the default setup as provided in the readme file. Do I need to do something special to have the proxy include the forms in the proxied requests?