Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 14431)
+++ wp-includes/post.php	(working copy)
@@ -2078,7 +2078,7 @@
 	if ( empty($post_category) || 0 == count($post_category) || !is_array($post_category) ) {
 		// 'post' requires at least one category.
 		if ( 'post' == $post_type )
-			$post_category = array( get_option('default_category') );
+			$post_category = array( 0 => get_option('default_category') );
 		else
 			$post_category = array();
 	}