R – Flex Type Coercion, it’s gone mental I think

I have a class essentially:

public class WindowEvent extends Event
{
    public static const WARNEVENT:String = "warnEvent";
    public static const TASKREQEVENT:String = "taskRequestEvent";
    public static const TASKANNOUNCE:String = "taskAnnounce";
    public static const WINDOWCHANGE:String = "windowChange";
    public static const ILLEGALPOSITION:String = "illegalPosition";

    // insert brevity   
}

The first four events were working fine, but I just I added ILLEGALPOSITION and tried this:

    // inside Window.as
    private function checkDimensions():void {
       if(!Window._legalBoundaryEnable)
           return;
...    var pass:Boolean = Window.legalBoundary.containsRect(
455        this.getBounds(stage));
456    if(!pass) {
457        this.dispatchEvent(new WindowEvent(WindowEvent.ILLEGALPOSITION,
...           "Illegal Position etc."));
       }
    }

So Flex spewed this stack at me when I hit the dispatch method:

TypeError: Error #1034: Type Coercion failed: cannot convert ¬
        flex.utils.ui::WindowEvent@511dce41 to flash.events.MouseEvent.
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent() ¬
        [C:\autobuild\~\UIComponent.as:9298]
    at flex.utils.ui::Window/checkDimensions()[C:\Users\~\Window.as:457]
    at flex.utils.ui::Window/stageResized()[C:\Users\~\Window.as:220]

As you can see from the trace, Window.as:457 is the last user code line. So WTF is flash.events.EventDispatcher.dispatchEventFunction trying to do with a MouseEvent?