使用Fiddler和基本身份验证来访问RESTful WebAPI

I have a WebAPI that works without issue. I have tested locally and deployed to my server and configured this service in IIS to use Basic Authentication. I am able to browse to my service and I receive the Authentication challenge I expect and all works swimmingly! Now I want to use Fiddler to test this and I have constructed a POST to a specific url and I got a 401 (Unauthorized) error. So I decided to add a base64 string in my Request Header and I am now getting a 500 error.I have a WebAPI that works without issue. I hav