The 'DelegatingHandler' list is invalid because the property 'InnerHandler' of 'handler' is not null
I have a custom message handler that I add globally as follows: public class CustomerHandler : DelegatingHandler { public CustomHandler(HttpConfiguration httpConfiguration) { InnerHandler = new HttpControllerDispatcher(httpConfiguration); } } config.MessageHandlers.Add(new CustomHandler(config)); However, I get the following error: The 'DelegatingHandler' list is invalid because the property 'InnerHandler' of 'CustomHandler' is not null. Parametername: handlers So I changed the code to: config.MessageHandlers.Add(new CustomHandler(config) { InnerHandler = null }); And I get an error: