[gentoo-user] TMux urxvt and Unicode

Hi,

I am using urxvt in conjunction with the terminus font without
problems.
When I call 

    pstree -G  -p

everything is fine.

Now I start tmux inside urxvt and do the same

    pstree -G  -p

and now all horizontal and vertical lines are no existing any more.
Crosspoints are drawn as '+'.

Changing to Ascii drawn lines is currently not an option.

As I understand tmux is nothing, which needs any encoding science,
since tmux is not an terminal emulator...or am I wrong here?

How can I make pstree make work in tmux with the above mentioned
options?

Thank you very much in advance for any help!
Best regards and have a nice weekend!
mcc