there must be at least one file in /etc/sudoers.d with permissions 0440,

[debian/sudo] / debian / NEWS

sudo (1.6.8p12-5) unstable; urgency=low

  The sudo package is no longer configured --with-exempt=sudo.  If you 
  depend on members of group sudo being able to run sudo without needing
  a password, you will need to put "%sudo ALL=NOPASSWD: ALL" in 
  /etc/sudoers to preserve equivalent functionality.

 -- Bdale Garbee <bdale@gag.com>  Tue,  3 Apr 2007 21:13:39 -0600