Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 15490)
+++ wp-includes/post.php	(working copy)
@@ -2673,7 +2673,7 @@
 			$post_categories = array( get_option('default_category') );
 		else
 			$post_categories = array();
-	} else if ( 1 == count($post_categories) && '' == reset($post_categories) ) {
+	} else if ( 1 == count($post_categories) && isset($post_categories[0]) && '' == reset($post_categories) ) {
 		return true;
 	}