Index: wp-admin/includes/post.php
===================================================================
--- wp-admin/includes/post.php	(revision 21049)
+++ wp-admin/includes/post.php	(working copy)
@@ -404,6 +404,9 @@
 function get_default_post_to_edit( $post_type = 'post', $create_in_db = false ) {
 	global $wpdb;
 
+	if ( ! post_type_exists( $post_type ) )
+		return new WP_Error( 'bad-post-type', sprintf( __( 'The specified post type “%s” does not exist.' ), $post_type ) );
+
 	$post_title = '';
 	if ( !empty( $_REQUEST['post_title'] ) )
 		$post_title = esc_html( stripslashes( $_REQUEST['post_title'] ));