Copy shapekey results in empty shape key

I set the shape key I want to copy all the way to 1. I then click the + to add a new shape key. Instead of being a copy, it is an empty shape key. Is there a step I may be missing?

Just what I was looking for! Thanks!

The + button doesn’t actually make a copy, nor there is a simple command to do that.
AFAIK to accomplish the copy you have to deactivate all the shape keys but the one you want to copy, then choose the command ‘New Shape from mix’ from the arrow pop up menu on the right.

EDIT: too late, grrr!!