BP SOcial, problem on activity page

Hi, i got a problem on activity page.
When a non logged in user go to activity page, this error message appears

Warning: Cannot modify header information - headers already sent by (output started at /home/public_html/wp-content/themes/bp-social/activity/index.php:12) in /home/public_html/wp-includes/pluggable.php on line 934

But for logged in users, ther is no problem..
This problem appears only in activity page