Mapping error using discriminator in NHibernate

If you are working with NHibernate and trying to use the discriminator feature, you may found an error saying that, according to the XML Schema validation, you can’t put a “discriminator” element under a “class” element. Which is very odd since that’s what the official documentation says:
http://nhibernate.info/doc/nh/en/index.html#mapping-declaration-discriminator