Uncaught TypeError: test1.change is not a function(…)

im learning more about javascript design patterns, so i been doing a example, in this case im creating a singleton, but im having some problems in one function called "change", is giving me a error saying the change is not a function, i dont understand why since is nothing difference from the others functions.

var singleton = (function(){

    var _counter;

    return{
        create: function (tagName, id){
            var el = document.createElement(tagName);
            el.id =  id || "Custom ID-"+this.generateID();
            return el;
        },
        generateID : function(){
            if(!_counter){
                _counter = 1;
                return _counter;
            }else{
                return _counter;
            }

        },
        change : function(id){
            _counter = id;
        }

    }


}());

Creating objects:

var test1 = singleton.create("div"); (output: <div id="Custom ID-1"></div>)
var test2 = singleton.create("span");(<span id="Custom ID-1"></span>
test1.change("2");