Index: wp-admin/includes/ajax-actions.php
===================================================================
--- wp-admin/includes/ajax-actions.php	(revision 19888)
+++ wp-admin/includes/ajax-actions.php	(working copy)
@@ -1530,6 +1530,7 @@
 }
 
 function wp_ajax_set_post_thumbnail() {
+	global $post_ID;
 	$post_ID = intval( $_POST['post_id'] );
 	if ( !current_user_can( 'edit_post', $post_ID ) )
 		wp_die( -1 );
@@ -1643,4 +1644,4 @@
 
 	update_user_meta( get_current_user_id(), 'dismissed_wp_pointers', $dismissed );
 	wp_die( 1 );
-}
\ No newline at end of file
+}