diff --git a/lisp/language/thai-util.el b/lisp/language/thai-util.el
index 5302a8d5a60..0c09b79afd4 100644
--- a/lisp/language/thai-util.el
+++ b/lisp/language/thai-util.el
@@ -229,7 +229,7 @@ positions (integers or markers) specifying the region."
   (thai-compose-region (point-min) (point-max)))
 
 ;;;###autoload
-(defun thai-composition-function (pos &optional string)
+(defun thai-composition-function (pos to font-object string)
   (setq pos (1- pos))
   (with-category-table thai-category-table
     (if string