Cannot create keyboard shortcut with percent character (% shift+5 shift+%)

The title pretty much explains it. I'm trying to modify my keymap so that it has some of the same shortcuts as my tmux setup. In particular, I'd like to be able to split the editor window vertically using a two-stroke combo: ^B, % (or ctrl+B, shift+%).

But the keymap editor won't record shift+% as a stroke, the second stroke field just stays empty:

But it doesn't have any trouble accepting any of the characters around %:

I'm running WebStorm 2017.2.4 on MacOS Sierra 10.12.6 (16G29), with a standard US Apple keyboard. I have IdeaVim installed as well.