Index: wp-includes/class-wp-customize-manager.php
===================================================================
--- wp-includes/class-wp-customize-manager.php	(revision 48360)
+++ wp-includes/class-wp-customize-manager.php	(working copy)
@@ -1907,7 +1907,14 @@
 		 * that the user's session has expired and they need to re-authenticate.
 		 */
 		if ( $this->messenger_channel && ! current_user_can( 'customize' ) ) {
-			$this->wp_die( -1, __( 'Unauthorized. You may remove the customize_messenger_channel param to preview as frontend.' ) );
+			$this->wp_die(
+				-1,
+				sprintf(
+					/* translators: %s: customize_messenger_channel */
+					__( 'Unauthorized. You may remove the %s param to preview as frontend.' ),
+					'<code>customize_messenger_channel<code>'
+				)
+			);
 			return;
 		}