Fatal error: Call to undefined function is_tree_allowed()

[qqtemp]

when in Graph-> Tree Mode
there is a error:

Fatal error: Call to undefined function is_tree_allowed() in /data/htdocs/cacti/graph_view.php on line 94

Redhat AS4
cacti-0.8.7b
php-5.2.6
httpd-2.0.53
mysql-5.0.22

how to solve this problem? Plz..