Index: wp-admin/async-upload.php
===================================================================
--- wp-admin/async-upload.php	(revision 39543)
+++ wp-admin/async-upload.php	(working copy)
@@ -52,7 +52,7 @@
 if ( isset($_REQUEST['attachment_id']) && ($id = intval($_REQUEST['attachment_id'])) && $_REQUEST['fetch'] ) {
 	$post = get_post( $id );
 	if ( 'attachment' != $post->post_type )
-		wp_die( __( 'Unknown post type.' ) );
+		wp_die( __( 'Invalid post type.' ) );
 	if ( ! current_user_can( 'edit_post', $id ) )
 		wp_die( __( 'Sorry, you are not allowed to edit this item.' ) );
 
Index: wp-admin/post.php
===================================================================
--- wp-admin/post.php	(revision 39543)
+++ wp-admin/post.php	(working copy)
@@ -107,7 +107,7 @@
 		wp_die( __( 'You attempted to edit an item that doesn’t exist. Perhaps it was deleted?' ) );
 
 	if ( ! $post_type_object )
-		wp_die( __( 'Unknown post type.' ) );
+		wp_die( __( 'Invalid post type.' ) );
 
 	if ( ! in_array( $typenow, get_post_types( array( 'show_ui' => true ) ) ) ) {
 		wp_die( __( 'Sorry, you are not allowed to edit posts in this post type.' ) );
@@ -208,7 +208,7 @@
 		wp_die( __( 'The item you are trying to move to the Trash no longer exists.' ) );
 
 	if ( ! $post_type_object )
-		wp_die( __( 'Unknown post type.' ) );
+		wp_die( __( 'Invalid post type.' ) );
 
 	if ( ! current_user_can( 'delete_post', $post_id ) )
 		wp_die( __( 'Sorry, you are not allowed to move this item to the Trash.' ) );
@@ -231,7 +231,7 @@
 		wp_die( __( 'The item you are trying to restore from the Trash no longer exists.' ) );
 
 	if ( ! $post_type_object )
-		wp_die( __( 'Unknown post type.' ) );
+		wp_die( __( 'Invalid post type.' ) );
 
 	if ( ! current_user_can( 'delete_post', $post_id ) )
 		wp_die( __( 'Sorry, you are not allowed to restore this item from the Trash.' ) );
@@ -249,7 +249,7 @@
 		wp_die( __( 'This item has already been deleted.' ) );
 
 	if ( ! $post_type_object )
-		wp_die( __( 'Unknown post type.' ) );
+		wp_die( __( 'Invalid post type.' ) );
 
 	if ( ! current_user_can( 'delete_post', $post_id ) )
 		wp_die( __( 'Sorry, you are not allowed to delete this item.' ) );
Index: wp-includes/class-wp-customize-nav-menus.php
===================================================================
--- wp-includes/class-wp-customize-nav-menus.php	(revision 39543)
+++ wp-includes/class-wp-customize-nav-menus.php	(working copy)
@@ -788,7 +788,7 @@
 	 */
 	public function insert_auto_draft_post( $postarr ) {
 		if ( ! isset( $postarr['post_type'] ) || ! post_type_exists( $postarr['post_type'] )  ) {
-			return new WP_Error( 'unknown_post_type', __( 'Unknown post type' ) );
+			return new WP_Error( 'unknown_post_type', __( 'Invalid post type.' ) );
 		}
 		if ( empty( $postarr['post_title'] ) ) {
 			return new WP_Error( 'empty_title', __( 'Empty title' ) );