Custom Java - Not working

I am trying to get the value of a field to display if its below 7.00

Although I keep getting an error every time I try to run it, but can't see where I'm going wrong?

Here's the code:

Function checkmargin()
{
    var margin = Xrm.Page.getAttribute("ro_margin").getValue()
    if (margin < 7.0)
    {
        //display the value within a warning box
        alert("Margin lower than Target", null, null, "WARNING", 500, 200);
    }
}