Index: src/main/java/hudson/plugins/active_directory/ActiveDirectoryAuthenticationProvider.java
===================================================================
--- src/main/java/hudson/plugins/active_directory/ActiveDirectoryAuthenticationProvider.java	(revision 15594)
+++ src/main/java/hudson/plugins/active_directory/ActiveDirectoryAuthenticationProvider.java	(working copy)
@@ -95,7 +95,11 @@
         } catch (ComException e) {
             throw new BadCredentialsException("Incorrect password for "+username);
         }
-
+        
+        if (usr== null){
+        	throw new UsernameNotFoundException("No such user: "+username);
+        }
+        
         List<GrantedAuthority> groups = new ArrayList<GrantedAuthority>();
         for( Com4jObject g : usr.groups() ) {
             IADsGroup grp = g.queryInterface(IADsGroup.class);