Actionscript 3 :: #2015: Invalid BitmapData

in my class, if I create bitmapData like this

private var tImage:BitmapData;
public function object():void {
tImage = new BitmapData(30,30,false,0x000000);
}

I get the following error

ArgumentError: Error #2015: Invalid BitmapData.

But if I declare the varible inside the method

public function object():void {
var tImage:BitmapData;
tImage = new BitmapData(30,30,false,0x000000);
}

It works fine.