I'm unable to open tmux in either iTerm2 or Terminal with an empty tmux.conf. Splitting windows from a running sessions also fails. (Not that I have any running sessions any more.) In iTerm2 I get one of three possible responses; with Terminal only two. (I've also tried the example tmux.conf that tmux ships with. Same results).
in iTerm2
[jayed@merlot ~/jank] tmux -f ./none.conf
[exited]
[jayed@merlot ~/jank] Term2 3.4.15
[jayed@merlot ~/jank] tmux -f ./none.conf
[exited]
[jayed@merlot ~/jank] tmux -f ./none.conf
^[[>0;95;0c^[P>|iTerm2 3.4.15^[\[exited]
in Terminal
[jayed@merlot ~/jank] tmux -f ./none.conf
[exited]
[jayed@merlot ~/jank] tmux -f ./none.conf
^[[>1;95;0c[exited]