Error with WebApi 2.0 RouteAttribute

Question

Here's my Global.asax

    protected void Application_Start()
    {
        RegisterRoutes();
    }

    private static void RegisterRoutes()
    {
        AreaRegistration.RegisterAllAreas();

        GlobalConfiguration.Configure(WebApiConfig.Register);
        FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
        RouteConfig.RegisterRoutes(RouteTable.Routes);

        GlobalConfiguration.Configure(x => x.MapHttpAttributeRoutes());
        GlobalConfiguration.Configuration.EnsureInitialized();
    }

Here's my Web Api controller

    [RoutePrefix("api/admin/users/")]
    public class UsersController : ApiController
    {
        [Route("get")]
        public IQueryable<User> GetUsers()
        {
            return db.Users;
        }
    }

And here's the error I get when I navigate to localhost:123/api/admin/users/get

The object has not yet been initialized. Ensure that HttpConfiguration.EnsureInitialized() is called in the application's startup code after all other initialization code.

I have no idea what I'm doing wrong here. I believe that I'm doing everything properly for the new WebApi 2.0 way, but I'm missing something.

Thanks in advance.

Update

Here's the stack in-case it helps

at System.Web.Http.Routing.RouteCollectionRoute.GetRouteData(String virtualPathRoot, HttpRequestMessage request) 
at System.Web.Http.WebHost.Routing.HttpWebRoute.GetRouteData(HttpContextBase httpContext)

Answer 1

Please remove

    GlobalConfiguration.Configure(x => x.MapHttpAttributeRoutes());

from Global.asax.

And then call MapHttpAttributeRoutes in the WebApiConfig.cs

    public static void Register(HttpConfiguration config)
    {
        config.MapHttpAttributeRoutes();
    }

Answer 2

In my case, I was getting this error because I was configuring Autofac before WebApi:

GlobalConfiguration.Configure(WebApiConfig.Register);
GlobalConfiguration.Configure(AutofacConfig.Register);

Changing the order got me past that issue:

GlobalConfiguration.Configure(AutofacConfig.Register);
GlobalConfiguration.Configure(WebApiConfig.Register);