[flexcoders] Error comparing bitmapData

[method_air]

I'm getting this error when comparing (what appears to be) two valid BitmapData 
objects, any ideas why?

Type Coercion failed: cannot convert -3 to flash.display.BitmapData.

code:

var diff:BitmapData =
BitmapData(this._editorBitmapData.compare(this._iceBitmapData));
                        
                
Cheers,

Philip