Emacs undo binding on OS X

Every time I hit ctrl-/ to undo, the default key binding for undo in the Emacs keymaps that come with PyCharm, the undo happens but I also get a system beep.  This beep doesn't happen if I use ctrl-underscore, the other default binding, or if I select undo from the "Edit" menu.

Why is PyCharm beeping whenever I undo with C-/?  How can I stop it from beeping?

PyCharm 3.4
Mac OS X 10.9.5
Apple JRE 1.6.0

Thanks!