Web api routing only permits one controller

I created a new Web API project and created the following routing spec (actually I have simplified, looking for the bug):

  // Web API configuration and services
  config.Routes.MapHttpRoute(
      name: "DefaultApi",
      routeTemplate: "api/{controller}/{action}/{id}"
   );

When I make a call via ajax with the route:

 /api/account/GetSUID/0 

everything works fine. When I just use a different controller:

 /api/tile/GetTileSet/0

it returns a status of 200 but instead of hitting my controller, it just returns the contents of the default page in jqXHR.responseText! It is as if it is just skipping any API routing like I am requesting the default site page.

I am baffled by this one as I have written literally hundreds of web API functions over the past few years in several other projects. I have never had any issue making calls to multiple controllers. I have looked high and low for what could be happening here and am hoping that someone here might have an idea.