swfObject 2.2 allowFullScreen does not work

Developing an ASP MVC site which has a Flash Intro and encountered a strange behavior: it is impossible to go into full screen (in any browser). Flex application has this script to go in full screen (AS3):

private function FullScreenOn(event:Event):void {
    this.stage.displayState = StageDisplayState.FULL_SCREEN;
}

and the HTML page which loads swfObject (2.2) has (amongst others) this code:

<script type="text/javascript">
           var flashvars = {};
            flashvars.foo = "bar";
            var params = {};
            params.allowfullscreen = "true";
            var attributes = {}; 

            swfobject.embedSWF(   "<%=Url.Content("~/Assets/PlayAround.swf")%>", 
                                  "Intro", 
                                  "100%", 
                                  "100%", 
                                  "9.0.28", 
                                  flashvars开发者_如何学Python, 
                                  params,
                                  attributes
                               );

        </script>

AFAIK this should work! but i'm getting an error stating

SecurityError: Error #2152: Full screen mode is not allowed.

What am i missing?

It looks to me like you are missing one of the arguments to embedSWF (the expressinstall url)

swfobject.embedSWF( swfUrl, 
                    id,
                    width,
                    height,
                    version,
                    expressInstallSwfurl,
                    flashvars,
                    params,
                    attributes, 
                    callbackFn
);