Hey,
recently I've switched to Windows and trying to adapt WSL (linux subsystem) to make it play together with my habits.
This time the issue was Tmux starting bash when I need ZSH. No matter what shell was set by `chsh -s` command Tmux was don't care.
The thing I found on stackoverflow is setting `default-shell` option of Tmux to `/bin/zsh` (in my case).
Here is what you need to add to your `~/.tmux.conf`:
set-option -g default-shell /bin/zsh
Don't forget to restart your Tmux or `:source-file` the new configuration.