[squid-users] squid

I've a linuxbox running slackware 8 and have recently compiled squid 2.5
stable20032013 with --enable-basic-auth-helpers=NCSA option.
In my squid .conf i have:
auth_param basic program /usr/local/squid/libexec/ncsa_auth
/usr/local/squid/etc/passwd
acl users proxy_auth "/usr/local/squid/acl/users.txt"
when i run squid -k reconfigure i get this message:
IGNORING: Proxy Auth ACL 'acl users proxy_auth
"/usr/local/squid/acl/users.txt"' because no authentication schemes are
fully configured.
2003/02/12 14:19:23| aclParseAclLine: IGNORING invalid ACL: acl users
proxy_auth "/usr/local/squid/acl/users.txt"
Can anybody help me?

-- 
Vinícius