Tmux' "Run-Shell" Blocks for Background Processes

I'm using tmux version 1.9a and I noticed that run-shell blocks using of any prefix-key commands until the command terminates.

To reproduce this just execute

run-shell "sleep 10"

and try to execute any of the prefix commands (e.g. C-b c to create a new pane). You will notice that nothing happens. Once the command terminates the action will be executed.

For me this behaviour is undesired because the command which I want to be execute should never terminate. Is this a bug or a feature? Furthermore, any ideas on how to solve this?

In case you're wondering about the concrete use-case: I'm trying to get a tmux + kerberos setup to work properly. For this I would like to use the ktmux wrapper which you can find here.