Set Route Attribute with a String Parameter

I'm attempting to create a Web API method with a custom route that takes in a String parameter, but this appears to error upon application start - so I don't even need to hit the route or the method for the application to break.

// Pass String to TMS and Return an ID
[System.Web.Http.HttpGet]
[System.Web.Http.Route("api/job/import/{import:string}")]
public int TmsImport(string import)
{
    var TmsId = 0;
    if (import.Length > 0)
    {
        TmsId = 1;
    }
    return (TmsId);
}

The application breaks on the following method in Global.asx:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    GlobalConfiguration.Configure(WebApiConfig.Register);
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}

This same method works when I pass an int to the method, but it doesn't seems to able to recognise the name 'string' in the parameter. I did see an answer to a similar question here: call to web api with string parameter - but I can't figure out where the change would be from that answer? If it would be in the call from the JS or in the route config?