MVC requests for /null returning 404 errors - asp.net-mvc

Question

I have an MVC project that appears to work great, unless you look at the error log. After every page is returned successfully, there is an attempt to load site.example.com/null and I have no idea why. Here's a snippet from Fiddler:
It has no effect to the user, but it is annoying. Here's a sample of the code that's being called:
public class GuidanceController : Controller
{
public ActionResult Index()
{
return View();
}
}
I have nothing unusual in the view and I haven't had to change the RouteConfig. I do have a custom authorization class, like so:
namespace MyProj.Filters {
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = true)]
public class MyProjAuthorizeAttribute : AuthorizeAttribute
{
public AccessLvl[] AccessLvls;
protected override bool AuthorizeCore(HttpContextBase httpContext)
{
AuthorizationClient authClient = new AuthorizationClient();
var authorized = base.AuthorizeCore(httpContext);
if (!authorized)
return false;
int intId = 0;
string requestId = httpContext.Request.Path.Split('/').Last(); //get ID sent with new page
string referrerId = httpContext.Request.UrlReferrer?.AbsolutePath.Split('/').Last(); //get ID sent with old page
if (int.TryParse(requestId, out intId) != true) //prefer new ID, if available
int.TryParse(referrerId, out intId); //else just use old id
List<AccessLvl> userAccessLevels = authClient.GetAccessLevels((intId == 0) ? null : intId.ToString());
foreach (AccessLvl level in AccessLvls)
{
if (userAccessLevels.Contains(level))
{
return true;
}
}
return false;
}
protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
{
filterContext.Result = new RedirectToRouteResult(
new RouteValueDictionary(
new
{
controller = "Error",
action = "Unauthorized",
urlReferrer = filterContext.RequestContext.HttpContext.Request.Url
})
);
}
}
}
What am I overlooking? Any ideas are greatly appreciated.