Index: languages/Messages.php
===================================================================
--- languages/Messages.php	(revision 14292)
+++ languages/Messages.php	(working copy)
@@ -395,6 +395,7 @@
 'loginsuccess'	=> "'''You are now logged in to {{SITENAME}} as \"$1\".'''",
 'nosuchuser'	=> 'There is no user by the name "$1". Check your spelling, or create a new account.',
 'nosuchusershort'	=> 'There is no user by the name "$1". Check your spelling.',
+'nouserspecified'	=> 'You have to specify a user.',
 'wrongpassword'		=> 'Incorrect password entered. Please try again.',
 'wrongpasswordempty'		=> 'Password entered was blank. Please try again.',
 'mailmypassword' 	=> 'E-mail password',
Index: includes/SpecialUserrights.php
===================================================================
--- includes/SpecialUserrights.php	(revision 14292)
+++ includes/SpecialUserrights.php	(working copy)
@@ -145,7 +145,10 @@
 		global $wgOut, $wgUser;
 
 		$user = User::newFromName($username);
-		if( is_null( $user ) || $user->getID() == 0 ) {
+		if( is_null( $user ) ) {
+			$wgOut->addWikiText( wfMsg( 'nouserspecified' ) );
+			return;
+		} elseif( $user->getID() == 0 ) {
 			$wgOut->addWikiText( wfMsg( 'nosuchusershort', wfEscapeWikiText( $username ) ) );
 			return;
 		}