Index: includes/api/ApiQueryRevisions.php
===================================================================
--- includes/api/ApiQueryRevisions.php	(revision 105848)
+++ includes/api/ApiQueryRevisions.php	(working copy)
@@ -131,7 +131,7 @@
 				if ( !$difftoRev ) {
 					$this->dieUsageMsg( array( 'nosuchrevid', $params['diffto'] ) );
 				}
-				if ( !$difftoRev->userCan( Revision::DELETED_TEXT ) ) {
+				if ( $difftoRev->isDeleted( Revision::DELETED_TEXT ) ) {
 					$this->setWarning( "Couldn't diff to r{$difftoRev->getID()}: content is hidden" );
 					$params['diffto'] = null;
 				}