It seems that this error only comes when the "login" attribute is "pam".
Actually, I have just tested the following conf file and it did not raise any error: ##################### working config file ################## key=something accounting file = /var/log/tac-plus/accounting user = $enab15$ { login = cleartext "spicegirls" } user = me1 { member = rw } user = me2 { member = ro } group = rw { default service = permit login = cleartext "hello" } group = ro { default service = permit login = nopassword } ########################################################