Iscroll Empty Space

I implemented iScroll successfully. But the problem is the screen scrolls even if there is no content.
Any suggests to solve this problem?
I am trying with that:

// enter code here
onScrollMove: function(e){
    var cont =$('.wrapper');
    alert($(cont).height());
    var docViewBottom = that.y + $(cont).height();
    if (docViewBottom<=0) {
        that.y=-($(cont).height())+30;
        that.startY = that.y;
        that.refresh();
        e.preventDefault();
    }
},

But the problem is every time the height is getting 426px. Is there any way to get the real height of the content?

I have solved by setting the screen height on the Iscroll Height