Writing a test in modernizr to check for overflow-y: auto support in a browser

I tested Modernizr and tried to write a test in it to check if the css property is supported in the browser.
Here I wrote for 'overflow-y: auto'

Modernizr.addTest('overflowauto', function(){
    var bool = false;
    var testProp = "overflow-y";
    var testVal = "auto";
    var styles = Modernizr._prefixes.join(testProp + ":" + testVal + "; ");
    var ret = true;
    ret =  Modernizr.testStyles('#modernizr { '+styles+' }', function(elem, rule){
        for(var i = 0; i < Modernizr._prefixes.length; i++) {
            bool = (window.getComputedStyle ?
                    getComputedStyle(elem, null) :
                    elem.currentStyle)[Modernizr._prefixes[i] + testProp] == testVal;

            if(bool) break;
        }

        return bool;

    });
    return ret;
});

If supported, this should set the value of Modernizr.overflowauto to true.

But the fact is that this always returns the truth. :(

even in android <3, where overflow-y: auto is not supported, its return is true. Pls tell me what i am doing wrong or how to achieve what i am trying. Pls help.