navigateToUrl - IE8 pop up block

I have a question:  The following code, which works OK in Firefox and Safari, gets IE8 to throw a pop up block

The simple code:

homeButton.addEventListener(MouseEvent.MOUSE_DOWN, smartlamp);

function smartlamp(event:MouseEvent):void

{
navigateToURL(new  URLRequest("http://www.hanegal.dk/Allergi_venlige_produkter.asp"),  "_blank");
}

example: http://www.allergiskhverdag.dk/TILBUD/hanegal/

Have I done something wrong here?

Round the internet I have found all sorts of workarounds - but eh.....  why?

using _blank on the other (html) links in my site works as expected without getting blocked

Suggestions are appreciated

Regards Henrik