AS3: declaring an "undefined" parameter for int, uint or Number

I'd like to implement the following logic:

function setMyValue (myVar:int = undefined):void
{
    if (myVar == undefined)
    {
        /* Generate a value for myVar */
    }
    else
    {
        /* Use the supplied value for myVar */
    }
}

So if the value is supplied, use it. If not, generate it. Seems simple enough.

Problem is that AS3 doesn't like this. 'undefined' gets coerced into '0', so myVar is never undefined. I've tried variations with NaN, null, etc, but nothing has worked. The only kludge I can think of offhand is to supply some 'magic number' to use as default, but that's ugly. Anyone know a better solution?

Thanks!