ActionScript 3.0 :: Removing A Number Value?

I'm creating a flash quiz that has drag-and-drop functionality. As part of this, I have 2 classes, one called Dragger and the other Dropper.

When Dragger is dragged above a Dropper, when I release the Dragger, it locks in place with the Dropper, and I set the value of the Dragger to the Dropper, using a setter function in the Dropper to update a value called _dropped.

So it looks like this in the main document class:

ActionScript Code:
public function dragKiller(e:MouseEvent):void {
// e is the MouseEvent associated with the Dragger
e.target.parent.stopDrag();

[Code].....

Now this is all mostly working fine for me. But what I want to do is say if I've previously dragged a Dragger onto a Dropper, and I remove that Dragger from the Dropper, and it returns to its home position (this currently works too). I want to remove that _dropped number reference from the Dropper.

When I initiated the _dropped variable in Dropper, I did it so there's no inital value, private var _dropped:Number; How do I remove that number value?

Currently, I'm passing a null here. Which is working, but throws a warning when I publish, and I have a feeling this is causing some bugs.

I don't want to set the _dropped value to zero, because my Dragger values are zero indexed, so I do have a Dragger with a value of zero, so that won't work.

I've tried using a negative number, but negative numbers aren't "Numbers" in AS3, which I need these to be.

how to remove a specific number value from a Number variable