[jQuery] form.submit not function error jquery form plugin

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I'm trying to get an upload popup working with the jQuery form plugin
(http://jquery.malsup.com/form/).

When I click a link I load a form html from the server and add it to a
container div by setting the div's html attribute.  I then attach a
submit handler to the form so I can call the ajaxSubmit function of
the form plugin.

$('#' + popup.popupId + ' form').submit(function(event) {
        event.preventDefault();

        var options = {
                dataType : popup.dataType,
                success : function (data, textStatus) {
                        popup.successHandler(popup, data, textStatus);
                }
        };

        $('#' + popup.popupId + ' form').ajaxSubmit(options);
});


The ajaxSubmit function however throws a "form.submit is not a
function" error.  I've tried debugging in firebug and form is an
object so I don't understand why the error.

Thanks for your help