folding let- declaration of Ocaml under Emacs gives "invalide search bound" error information

Under Emacs, I am trying to fold ocaml source files in such a way that:

each let-definition is folded into something like "let myfunction = ..."

This should be a natural request for all ocaml programmes. I think the answer for Emacs should be the outline-minor-mode. But for whatever command related to outline-mode, I got the same error info. from the minibuffer

invalide search bound(wrong side of point)

For info, I am under Tuareg major mode for ocaml editing. I activate outline-minor-mode by

M-x outline-minor-mode

then, I try to hide sublevel开发者_如何学运维s by

M-x hide-sublevels

which gives me the above mentioned error info. Any ideas?