Integration of Prezi in modal window (bootstrap) doesn't work, why ?

Hello,

   I try to integrate my Prezi in a modal window Bootstrap but iframe is empty.

Firebug indicates "Error: Permission denied to access property 'toString' "
Yet i add toString() function :

function seeHelp(name){

       var src = "http://prezi.com/embed/xvwqvl4k5odf/?bgcolor=ffffff&lock_to_path=0&autoplay=0&autohide_ctrls=0&features=undefined&disabled_features=undefined".toString();

    $("#helpModalFrame").attr({'src':src,'height': 300,'width': 200});

$('#helpModal').modal('show') ;

}

And my modal body :

<div class="modal-body"><iframe id="helpModalFrame"> </iframe></div>

When I move my iframe outside the modal, Prezi works well ...
Do you have ways to solve this problem?
Thank you!