Why GetBoundingRect isn't instance of cr.rect

cr.rect.prototype.height = function () {
    return Math.abs(this.top - this.bottom)
}[/code:2jj9qpq9]
But I can't access this method from result of [i]instance.GetBoundingRect()[i]

I wrote this test code:
[code:2jj9qpq9]var x = 0;

IDEInstance.prototype.Draw = function(renderer)
{
    var rect = this.instance.GetBoundingRect()
    if (x == 0) {
        alert(rect instanceof cr.rect);
        x = 1;
    }
}[/code:2jj9qpq9]
And it alerted false   while in documentation is written:
[quote:2jj9qpq9]instance.GetBoundingRect()
Return a cr.rect object specifying the object's axis-aligned bounding rectangle.