Index: ActiveDirectoryAuthenticationProvider.java
===================================================================
--- ActiveDirectoryAuthenticationProvider.java	(revision 16504)
+++ ActiveDirectoryAuthenticationProvider.java	(working copy)
@@ -95,7 +95,10 @@
         } catch (ComException e) {
             throw new BadCredentialsException("Incorrect password for "+username);
         }
-
+        // If username is in fact a group
+        if (usr == null) {
+        	throw new UsernameNotFoundException("User not found: "+username);
+        }
         List<GrantedAuthority> groups = new ArrayList<GrantedAuthority>();
         for( Com4jObject g : usr.groups() ) {
             IADsGroup grp = g.queryInterface(IADsGroup.class);