[O] Exporting in html with numeric references to headlines

[Shérab]

Dear all,

Assume the following document:

===

#+title: test1

* Introduction

Monads will be explained in chapter [[#monads]]

* Monads
  :PROPERTIES:
  :CUSTOM_ID: monads
  :END:

Welcome to the monads chapter.

===

When I export this to html, the paragraph in the introduction says:

Monads will be explained in chapter 2

with "2" being a link, which is what I am expecting. However the link
does not work and it seems the "#monads" anchor is actually not created.
I can also observe that the links in the table of contents do not seem
to work either.
Am I doing something wrong? I also tried different variations with
<<chap:monads>> but was not able to make anything work.

Many thanks in advance for any hint!

Shérab.