Button Event Handling AS3.0

[karlectomy]

Hi, I just recently started working with Flash CS3 and decided to try my fate in AS3. I did my research and read my sources and still I am having a problem with event handling on a button.
I want to have dynamic hover text on a series of buttons. Below I have implemented one of the buttons and I get this error at runtime:
TypeError: Error #2007: Parameter listener must be non-null.
at flash.events::E ventDispatcher/addEventListene r()

I understand the error, it thinks that onHoverEnter() is null but I have debugged and it seems not to be null to begin with. Please help.

Thanks,

Karl

 
lwrist.addEventListener(MouseEvent.MOUSE_OVER, captionFN(true, lwsttext, lwrist));
lwrist.addEventListener(MouseEvent.MOUSE_OUT, captionFN(false, null, null));
 
var xo:int;
var yo:int;
 
function captionFN(showCaption:Boolean, captionText:String, bName:SimpleButton) {
 
    if (showCaption) {
        var hoverCaption:MovieClip = new MovieClip();
        MovieClip(root).addChild(hoverCaption);
        desc.capdesc.text = captionText;
        desc.width = 7*desc.capdesc.text.length;
        desc.alpha = 75;
        //
        if ((bName.width+bName.x+desc.width)>stage.width) {
 
            xo = -2-desc.width;
            yo = -17;
 
        } else {
            xo = 2;
            yo = -17;
        }
        hoverCaption.addEventListener("enterFrame", onHoverEnter);
        var onHoverEnter = function(ev:Event):void {
            desc.x = root.mouseX+xo;
            desc.y = root.mouseY+yo;
            desc.visible = true;
        }
    } else {
        hoverCaption.removeEventListener("enterFrame", onHoverEnter);
        desc.visible = false;
    }
 
}