[Solved] Shape key buttons always disabled ?

Surprisingly, I cannot create any shapkey. Thus “+” button is always disabled.

Is it a known issue ?

Step : create any mesh, go to edit mode, select any vertice, open the shape key control : it is disabled.

Occurs on Blender 2.53.1

Too long I did not use Blender, I’ve forgot about many things : have to be in Object mode to create a shape key.