PAM pwdb "structure not valid" messages

[B Wooster]

In /var/log/messages, I get these lines:
PAM_pwdb ... get passwd; pwdb: structure is no longer valid

when a failed login takes place on a FTP session.

I don't see that line when a valid username/password is used with FTP.
Which leads me to believe that the shadow/passwd file is all fine, is
this is a config issue or a bug?

In /etc/pam.d/ftp file, if I comment out this line:
auth       required     /lib/security/pam_pwdb.so shadow nullok
then I don't get the "stucture no longer valid" messages (but of
course, PAM is now out of the picture).

I'm running FC3, all latest versions using "yum update", and proftpd
is my FTP server.