Index: includes/Wiki.php
===================================================================
--- includes/Wiki.php	(revision 59772)
+++ includes/Wiki.php	(working copy)
@@ -182,11 +182,13 @@
 		global $wgContLang, $wgUser;
 		$action = $this->getVal( 'Action' );
 		$perferred = $wgContLang->getPreferredVariant( false );
-		// Invalid titles
-		if( is_null($title) || $title->getDBkey() == '' ) {
+
+		// Invalid titles. Bug 21776: The interwikis must redirect even if the page name is empty.
+		if( is_null($title) || ( ($title->getDBkey() == '') && ($title->getInterwiki() == '') ) ) {
 			$title = SpecialPage::getTitleFor( 'Badtitle' );
 			# Die now before we mess up $wgArticle and the skin stops working
 			throw new ErrorPageError( 'badtitle', 'badtitletext' );
+
 		// Interwiki redirects
 		} else if( $title->getInterwiki() != '' ) {
 			if( $rdfrom = $request->getVal( 'rdfrom' ) ) {
Index: RELEASE-NOTES
===================================================================
--- RELEASE-NOTES	(revision 59776)
+++ RELEASE-NOTES	(working copy)
@@ -669,6 +669,7 @@
 * (bug 21559) "logempty" message is now wrapped in a div with class
   "mw-warning-logempty" when used in log extract
 * (bug 20549) Parser tests were broken on SQLite backend
+* (bug 21776) Interwiki urls like http://en.wikibooks.org/wiki/cs: should give a redirect instead of a baderror.
 
 == API changes in 1.16 ==