--- comment.php	2017-05-04 14:44:59.216212770 +0100
+++ comment-amended.php	2017-05-04 14:44:00.816215294 +0100
@@ -3069,6 +3069,7 @@
 	}
 
 	if ( '' == $comment_content ) {
+		do_action("require_valid_comment", $comment_post_ID);
 		return new WP_Error( 'require_valid_comment', __( '<strong>ERROR</strong>: please type a comment.' ), 200 );
 	}