ESC-q doesn't work in tmux with zsh

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http://unix.stackexchange.com – The title says it all. I use ESC-q a lot (stashing the current line while I execute another command), and it works fine with Gnome Terminal 3.6.0 + zsh. However, if I start tmux, then it stops working: the cursor just moves one character to the left and stays there. I don't set TERM in .zshrc, in .tmux.conf I use set -g default-terminal "screen-256color". (HowTos)