I have set -g base-index 1 in my tmux.conf, which makes window numeration start from 1. However, tmex still thinks it starts at zero, thus trying to do something with the 0th window in multi-window setup, producing a status bar error "can't find window: 0". I thought it was trying to focus it and tried -W 1 but it didn't change anything.
examples:

tmex ugh -w- "" -w- ""

tmex ugh -W 1 "" -w 2 ""

edit: well it's quite obvious why doesn't -W 1 fix that. 1 here is a window name, not a window number