P: LR4 Unable to take a snapshot of a virtual copy of an image starting from the History

Here is way to reproduce it:
I'm experiencing it on Adobe Lightroom 4 on Lion OSX

Go to the Develop module.
Select a picture and create a virtual copy of it.
Select the virtual copy you just created.
Play a bit, for example, with the tone curve many times so you create 5 or 6 elements of history.
Click (for example) in the middle of your History to select a particular ''action'' as for example "Point Curve: Custom"
Now right click and from the menu' choose the action Create snapshot...

Enter a name for the snapshot

RESULT: No snapshot has been created at all

FURTHER TESTS:

Click now on the + sign in the pane and name a snapshot

RESULT:

The snapshot is created.

Thanks & Regards,

Marco