emacs, switch-window, evil, and wrong type argument

When using emacs and the switch-window package, I occasionally got an error when trying to switch to a different window (C-x o):

Wrong type argument: stringp, nil

I tracked down the issue to evil mode key bindings. Basically, my qwerty shortcut keys were clashing with my evil movement keys.

My original switch-window keys (note the j, k, l):

(setq switch-window-qwerty-shortcuts
    '("a" "s" "d" "f" "j" "k" "l"))

My modified switch-window keys and we’re good to go:

(setq switch-window-qwerty-shortcuts
    '("a" "s" "d" "f" "w" "e" "r"))