I am getting has no method error

here is the sample code

```
var snake = function(){

    this.initialize = function(){
        // initalize
    };
    this.start = function() {
        this.initialize();
    };

};

var s = new snake();

var startButton = document.getElementByID("start");

startButton.addEventListener('click',s.start,false);

```

So when I click start button I am getting 

> 
> Uncaught TypeError: Object # has no method 'initialize' 
> 
> 
> 

how can i fix this