Suppress the visual/audible bell in Mail when I press ⌥ (option) plus a symbol

I've configured BetterTouchTool to allow me to toggle read/unread status of an email when I press ⌥/ (option slash, a convenient shortcut near the arrow keys), using the "Trigger menu bar menu item" action. (See below screenshot.)

The shortcut works—when I trigger it, the read/unread status changes. (BTT is smooth as glass, I LOVE it.)

However, Mail.app also makes an audible and visual bell (screen flash and a sound) when I trigger the shortcut. It does this whether or not BTT is running or configured.

Is there some way I can suppress this bell sound and flash for this particular keyboard shortcut for Mail?

(I'm running Mail 9.3 on macOS 10.11.6, and BetterTouchTool 1.912 (523).)

P.S.: I tried several other unbound keyboard shortcuts, without configuring them in BTT but just pressing them in Mail, and all made the same flash and sound. I have no reason to think they will not make the same annoying sound if configured to actually do something through BTT.