[horde] DB Error: extension not found ???

[Mehmet Fatih Akbulut]

hi all,
when i click on the Tasks, i get this error :

A fatal error has occurred
DB Error: extension not found

[line 459 of /var/www/horde/nag/lib/Driver/sql.php]
.
.
which package do i have to install ? not to get this error any more ?