“Value cannot be null or empty. Parameter name: co

It's been two hours straight I have been haggling with this monstrosity.

I get this error:

Value cannot be null or empty. Parameter name: contentPath

On this line in my view:

@Html.ValidationMessageFor(model => model.IssueName, 
  "", new { @class = "text-danger" })

The stack trace given below seems to suggest that a call to Url.Content has been made, but I have made no such call. Below is the stack trace and that is followed by some more lines of code around the line which causes the error:

[ArgumentException: Value cannot be null or empty. Parameter name: contentPath] System.Web.Mvc.UrlHelper.GenerateContentUrl(String contentPath, HttpContextBase httpContext) +125
System.Web.Mvc.UrlHelper.Content(String contentPath) +26
ASP._Page_Views_Journal_EditIssue_cshtml.Execute() in MyProject\Views\Journal\EditIssue.cshtml:45
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +105
System.Web.WebPages.StartPage.RunPage() +17
System.Web.WebPages.StartPage.ExecutePageHierarchy() +64
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +78
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +256
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +107
System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +291 System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +13
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +56 System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +420
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList1 filters, ActionResult actionResult) +52 System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +173 System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +100 System.Web.Mvc.Async.WrappedAsyncResult1.CallEndDelegate(IAsyncResult asyncResult) +10
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49 System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +27 System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +13 System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +29
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49 System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +36 System.Web.Mvc.Controller.<BeginExecute>b__15(IAsyncResult asyncResult, Controller controller) +12 System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +22
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49 System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +26 System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10 System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +21 System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +29
System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +28 System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9644097 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

The lines of code around the line that throws the exception are:

<div class="form-group">
    @Html.LabelFor(model => model.IssueName, htmlAttributes: new { @class = "control-label col-md-2"})
    <div class="col-md-10">
        @Html.EditorFor(model => model.IssueName, new { htmlAttributes = new { @class = "form-control", id = "IssueName" } })
        @Html.ValidationMessageFor(model => model.IssueName, "", new { @class = "text-danger" })
    </div>
</div>

IMPORTANT

Strangely, the exception is raised only when I come back from a post-back and never when I first load the view, and not even when the ModelState has no errors. It only is raised when the ModelState has errors after a post-back.

Here is some server side code, though I don't see any reason why it might be doing something wrong:

[HttpPost]
public async Task<ActionResult> EditIssue(EditIssueViewModel viewModel)
{
    viewModel.AvailableTags = BusinessManager.GetAllTags();

    if (viewModel.IssuePDFFile == null || viewModel.IssuePDFFile.ContentLength == 0)
    {
        ModelState.AddModelError("", "Please select a file to upload.");
        return View(viewModel);
    }

    var fileInfo = new FileInfo(viewModel.IssuePDFFile.FileName);
    if (!StaticData.AcceptedContentTypes.Contains(viewModel.IssuePDFFile.ContentType, StringComparer.InvariantCultureIgnoreCase) ||
        !fileInfo.Extension.Equals(".pdf", StringComparison.InvariantCultureIgnoreCase))
    {
        ModelState.AddModelError("", "You can only select a PDF file.");
        return View(viewModel);
    }

    if (!ModelState.IsValid)
    {
        var errors = ModelState.Values.SelectMany(v => v.Errors)
            .Select(e => new { e.ErrorMessage, e.Exception });

        var errorList = errors.ToList();

        errorList.ForEach(e => Debug.Print(e.ErrorMessage));
        errorList.ForEach(e => ModelState.AddModelError("", e.ErrorMessage));

        return View(viewModel);
    }

    var operationResult = await BusinessManager.EditIssueAsync(viewModel);

    if (!operationResult.Succeeded)
    {
        ModelState.AddModelError("", operationResult.FailureMessage);
        return View(viewModel);
    }

    viewModel = (EditIssueViewModel)BusinessManager.GetIssueWithRelationships(viewModel.IssueId);
    viewModel.SuccessMessage = operationResult.SuccessMessage;
    return View(viewModel);
}

Holy crap!

If you ever, ever, ever get this error, please remember, there is but one, and only one, cause for this error.