Executing Javascript elementFromPoint through Selenium driver

I am trying to implement an "object picker" to my Selenium based framework as is common on most commercial automation tools. To do this I am using a Javascript command to find the element at the mouse location, but I am not getting the element I expect.

If I am using ChromeDriver or InternetExplorerDriver the script always returns the header object. No matter what webpage I look at or the position of the mouse. Although it sounds like the script is taking the coordinates 0, 0 instead of the mouse position I have confirmed that Cursor.Position is sending the correct values.

If I am using FirefoxDriver I get an exception:

"Argument 1 of Document.elementFromPoint is not a finite floating-point value. (UnexpectedJavaScriptError)"

Can anyone see what I am doing wrong?

    private void OnHovering()
    {
        if (Control.ModifierKeys == System.Windows.Forms.Keys.Control)
        {
            IWebElement ele = null;
            try
            {
                // Find the element at the mouse position
                if (driver is IJavaScriptExecutor)
                    ele = (IWebElement)((IJavaScriptExecutor)driver).ExecuteScript(
                        "return document.elementFromPoint(arguments[0], arguments[1])", 
                        new int[] { Cursor.Position.X, Cursor.Position.Y });

                // Select the element found
                if (ele != null)
                    SelectElement(ele);
            }
            catch (Exception) { }
        }
    }

Thanks!