Lightroom 4.2 : Proofing copy alert popup problem

Hello,

I find a problem, simple to reproduce with soft proofing.

Select a photo in Library module
Create a new virtual copy of it
Open this virtual copy in Develop module
Go into soft proofing view
Use menu : "develop" and check (by clicking) : "proof copy"

The button "create a proof copy" bellow the histogram disappear as excepted.

Moving any slider popups an alert to create a "proof copy" for this setting. This should not happen.

I don't have the exact name of menu setting and button because I have a french interface.