Building custom OData feed for consumption in Powe...

I have a product that exposes an OData feed but requires authentication. That feed can be consumed in Power BI with no problems.

I constructed an ASP.NET MVC Web API application to act as a wrapper around that existing feed, managing the authentication in the user's behalf. The results from the wrapper look almost identical to the results from the feed itself, except for some whitespace differences in the response body that result in a different content length. However, Power BI complains when I try to target the wrapper as an OData Feed. I'm getting the following error.

The content type is specified in the MVC wrapper as application/json and the content of the response itself actually is JSON. I know Power BI supports that MIME type because the original feed returns the same content and it is consumed just fine.

Any ideas on where to look?