Actionscript 3 :: Merging Two Bitmapdata Objects' Alpha Channels?

i have the following situation:

one drawing layer (graphics object with lineTo, etc.)

one png with an alpha channel (supposed to serve only as a mask)

now i want to be able to only draw lines within an area restricted by the png mask.

i am trying like this:

var bitmapData:BitmapData = new BitmapData( 320, 320 );
bitmapData.draw( drawingLayer );
bitmapData.copyChannel( maskBitmapData, new Rectangle( 0, 0, 320, 320 ), new Point( 0, 0 ), BitmapDataChannel.ALPHA, BitmapDataChannel.ALPHA );

now the borders get cut off fine, but i get a black background, since the drawing layer has an alpha between the drawn lines (and it should remain like this) and the mask bitmap has an alpha outside the shape. so naturally the mask's alpha replaces the drawing layer's.

i tried it with merge, copyPixels and also with just setting the mask property on the drawing layer (i set everything to cacheAsBitmap) but to no avail.

ADDITION:

when trying to use a mask i tried it with

<s:BitmapImage id="mask" source="@Embed(source='...')" cacheAsBitmap="true" />

and also as

[Embed("...")]
private const BodyMask:Class;
var maskBitmap:Bitmap = new BodyMask();

and assigned them to the mask property of a s:Group element (cacheAsBitmap=true) where the operations on the graphics object occured. i also tried reassigning the mask after each draw operation.