Why "ldap_sasl_interactive_bind_s: No such attribute"?

["hbgui" <hbgui@student.dlut.edu.cn>]

I use ldapmodify cann't bind to ldap server.
messsage is:
cvs:~# ldapmodify -h 127.0.0.1 -D cn=laomao,dc=net,dc=dlut,dc=edu,dc=cn -w test123 -v   
ldap_init( 127.0.0.1, 0 )
ldap_sasl_interactive_bind_s: No such attribute

But I can success change it by use -x option.what's wrong with it?
I use pam_ldap cann't change password too.