Caused by missing terminfo entries for the terminal being used. Sometimes this can be fixed by upgrading ncurses or terminfo to a later version. In the event that it can't:
- Copy the terminfo entry from a machine that has it:
ssh <user@host> infocmp -x tmux-256color >~/tmux-terminfo
- Add it to your machine's terminfo database with:
tic -x ~/tmux-terminfo