set parameter using variable in tinyMCE

Question

I'm trying to create a plugin for tinyMCE that opens a new dialog window.

The URL that it opens depends on a variable as follows:

var open_url = 'http://www.mydomain.com/footnote/edit/'+ID;
tinyMCE.activeEditor.windowManager.open({
    url : open_url,
    width : 320,
    height : 240
});

If I just enter the URL parameter directly as a string, it works fine.

But as soon as I try to pass it a variable, it breaks. Any idea why?