--- post-template.php.ori	2010-01-05 22:43:53.390625000 +0100
+++ post-template.php	2010-01-09 11:40:56.421875000 +0100
@@ -646,7 +646,9 @@
 function post_custom( $key = '' ) {
 	$custom = get_post_custom();
 
-	if ( 1 == count($custom[$key]) )
+	if ( !isset( $custom[$key] ) )
+		return '';
+	elseif ( 1 == count($custom[$key]) )
 		return $custom[$key][0];
 	else
 		return $custom[$key];