(tibetan-pre-write-conversion): Make it work

for the case the arg FROM is a string.

lisp/language/tibet-util.el

@@ -444,9 +444,13 @@ See also docstring of the function tibetan-compose-region."
 	(work-buf (get-buffer-create " *tibetan-work*")))
     (set-buffer work-buf)
     (erase-buffer)
-    (insert-buffer-substring old-buf from to)
+    (if (stringp from)
+	(insert from)
+      (insert-buffer-substring old-buf from to))
     (if (not tibetan-decomposed-temp)
-	(tibetan-decompose-region (point-min) (point-max)))))
+	(tibetan-decompose-region (point-min) (point-max)))
+    ;; Should return nil as annotations.
+    nil))
 
 (provide 'language/tibet-util)