jQuery custom function not working on master page

I have created a custom jQuery function in my web application and it is working fine without master pages, now I have a master page and i have added a reference to my .js file on the Content page and call the custom js method then it gives me error like
 

Type error: object function (selector, context){ //the jquery object is actually just the constructor 'enhanced' return new jQuery.fn.init(selector, context, rootjQuery); }has no method 'myCustomMethod'

i am calling the custom method like:

  $(document).ready(function () {
            $('#btnTest').click(function () { 
                try {
                    $.myCustomMethod('testing title!', 'popup.aspx', 600, 400);
                } catch (e) {
                    alert(e);
                }
            });
        });

my js code is
 

(function ($) { 
$.fn.myCustomMethod = function (title, src, width, height) {
 // all my code here }; 
})(jQuery);

 
any help would be greatly appreciated.