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();
                 } catch (e) {
                     alert(e);
                 }
             });
         });

my js code is

(function ($) {
    $.fn.myCustomMethod = function () {
       // all my code here
}; })(jQuery);

any help would be greatly appreciated.