Shadowbox opens in _blank when width and/or height are set

Question

I've been searching everywhere on the internet now but I can't find a solution to my problem.

I'm using the shadowbox for my galleries and it's working perfectly fine if I don't use a fixed height or/and width. If I set one or both of these attributes the link opens in a _blank page.

This is what my code looks like

<div id="imprint">
   <a href="subpages/imprint.html" rel="shadowbox;width=645" title="Imprint">
        Imprint
   </a>
</div>

thanks for your replies mlp

update:

<script type="text/javascript">

$(document).ready(function(){

    var options = {
        resizeLgImages:     true,
        displayNav:         false,
        handleUnsupported:  'remove',
        keysClose:          ['c', 27] // c or esc
    };

    Shadowbox.init(options);

});

</script>