Javascript – why is the window.dialogArguments undefined

I need to open up a .aspx page in a modal dialog. Here is the JS code I use to open the dialog:

        if (url) {
            var fullPath = url + "/Validation.aspx";
        }
        else {
            alert("Could not find the location of the merge dialog.  Please contact your System admin and have them update the configuration entity.");
            return;
        }

        var features = "unadorned:yes;scroll:yes;resizable:yes;status:yes;dialogHeight:480px;dialogWidth:480px;";

        var args = {
            selected: selectedIds,
            page: pageIds,
            fetchXml: xml,
            entity: "xyz"
        };

        window.showModalDialog(fullPath, args, features);

In my validation.aspx page I need to be able to grab the JS arguments, assign them to hidden fields, then repost, so I can use those arg values server side.

here is my JS code in my .aspx page:

window.onload = function(){        
    if (!window.dialogArguments)
        return;

        var args = window.dialogArguments;
        ...
}

I have seen tons of examples of this working throughout the web. But…My window.dialogArguments is always undefined in my .aspx page. What gives? anyone have any thoughts or solutions?