js : how i can extend basic Array implementation in ie7?

I have a problem with extending basic Array implementation in IE7

function extend(Child, Parent){
   var F = function() { };
   F.prototype = Parent.prototype;
   Child.prototype = new F();
   Child.prototype.constructor = Child;
   Child.superclass = Parent.prototype
}

var MyArray = function(){};
extend(MyArray, Array);

var a = new MyArray();
a.push("value");
console.log(a.length);

in all browsers a.length == 1, but in ie7 a.length == 0

please help to resolve this problem