[away3d] BitmapMaterial mipmap

[Georgi Gevorgyan]

hi. I'm new in away3d platform. I'm using alpha release of broomstick.
I have  got a problem with mipmap. I'm loading external image and want
to make a BitmapMaterial like this:

class extends Plane
and here is function when load completes

private function comp(e:Event):void
{
        var a:BitmapData=new BitmapData(_width,_height);
        a.draw(ldr);
        bitMap=new BitmapMaterial(a,true,false,false);
        this.material=bitMap;
}

where _width, _height >0
but I'm getting error:
Error: Invalid bitmapData! Must be power of 2 and not exceeding 2048

so why I can't set bitmapData width and height any value I want ?

please help,
thanks.