null parameter in url

Question

I have some javascript that calls an action method on a controller (MVC, C#) there are two parameters defined, both have values but I get a null reference exception. Heres the javascript that calls the action method

$("*[id^=popout1]").on("click", function () {
            //debugger;
            $(".FakeClass").dropdown('toggle');
            var id = this.getAttribute("data-id");
            var reportType = $('#ReportExecutionType').val();
            var url = '@Url.Action("PopUp", "Benchmark", new { cid = "_id_" , reportName = "_ret_"})'.replace('_id_', id).replace("_ret_", reportType);
            newwindow = window.open(url, 'name', 'height=800,width=850');
            if (window.focus) { newwindow.focus() }
            return false;
        });

and this is the url thats generated

"/Benchmark/PopUp?cid=113&reportName=17"

this is the c# action method

[LayoutInjecter("_EmptyLayoutPage")]
    [HttpGet]
    public async Task<ActionResult> PopUp(int cid, Helpers.EnumHelper.Reports reportName)
    {
        return View("PopUp", await PopulateResponse(cid, reportName));
    }

but I get this error

The parameters dictionary contains a null entry for parameter 'reportName' of non-nullable type 'Common.Helpers.EnumHelper+Reports' for method 'System.Threading.Tasks.Task`1[System.Web.Mvc.ActionResult] PopUp(Int32, Reports)' in 'Controllers.Benchmark.BenchmarkController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter. Parameter name: parameters

I cant figure this one out, the parameter isn't misspelled and it has a value, so why am I getting this error ?