title: tmux won't start if default shell is fish
body: <p>I have <code>tmux</code> and <code>fish</code> both installed via <code>apt</code> on an Ubuntu 19.04 (Disco Dingo).</p>
  
  <p><code>tmux</code> works well out of the box but it uses <code>bash</code> and I would like it to use <code>fish</code> as default shell.</p>
  
  <p>Thus I put the following line in <code>~/.tmux.conf</code>:</p>
  
  <pre><code>~$ cat .tmux.conf
  set-option -g default-shell fish
  </code></pre>
  
  <p>But then <code>tmux</code> won't start anymore, instead it prints weird characters and dies:</p>
  
  <pre><code>~$ tmux
  [exited]
  ~$ [?65;1;9c
  </code></pre>
  
  <p>(Note: the weird characters are not typed by me, they end up here before my cursor)</p>
  
  <p>Note that if I remove <code>~/.tmux.conf</code>
  (then <code>tmux</code> starts using bash and works well),
  I am able to use fish by calling <code>fish</code> from <code>bash</code>.</p>
  
  <p>Any idea what is happening and how I could solve this?</p>