OS: Debian 9.0
tmux version: 2.3-4

The problem seems to be with these lines of code:
# Vi copypaste mode set-window-option -g mode-keys vi if-shell "test '\( #{$TMUX_VERSION_MAJOR} -eq 2 -a #{$TMUX_VERSION_MINOR} -ge 4 \)'" 'bind-key -Tcopy-mode-vi v send -X begin-selection; bind-key -Tcopy-mode-vi y send -X copy-selection-and-cancel' if-shell '\( #{$TMUX_VERSION_MAJOR} -eq 2 -a #{$TMUX_VERSION_MINOR} -lt 4\) -o #{$TMUX_VERSION_MAJOR} -le 1' 'bind-key -t vi-copy v begin-selection; bind-key -t vi-copy y copy-selection'