Chroot Eviroment with proftpd and pam us

Hello,
I got a Problem running proftpd in a chroot enviroment using pam and s/key.
The s/key pam modul don’t work.
withe the pam_unix module everything is O.K.
when changing setting in pam.conf for s/key
i got an error: Pam : symbol not found
Please help