Not circumcised when using Vac Extender.

Hi guys, I have been using my Master Pro Penis Extender, it’s very easy and comfortable to use, but lately I have experienced some swelling at the tip of the foreskin due to not being circumcised. I try to peel it prior to put it into the cup, but after I perform the suction, the foreskin kind of move half way to the glans area, Any idea on how to avoid any swelling?

Thanks for all the help.