In case if routeData contains null value of any key then RouteDataValueProvider thrown an exception NullReferenceException.

Fix:

yield return new KeyValuePair<string, string>(pair.Key, pair.Value != null ? pair.Value.ToString() : null);

-- source code --

public class RouteDataValueProvider : NameValuePairsValueProvider
{
    public RouteDataValueProvider(HttpActionContext actionContext, CultureInfo culture)
        : base(() => GetRoutes(actionContext.ControllerContext.RouteData), culture)
    {
    }

    internal static IEnumerable<KeyValuePair<string, string>> GetRoutes(IHttpRouteData routeData)
    {
        foreach (KeyValuePair<string, object> pair in routeData.Values)
        {
            yield return new KeyValuePair<string, string>(pair.Key, pair.Value.ToString());
        }
    }
}