help with su -l

Hi,
I'm rather new in Linux. So i apologise for , maybe very simple question,
the problem is:
root@host> su -l username -c ls
-bash : /proc/3769/fd/0; Permission denied;
{ normal output of ls command from /home/user_name}
Then I log to host via telnet all work OK.
Wht is wrong?
RH 6.