Mercurial hg up --clean --rev default does not update a sub repository

Problem:
when using a 'super' repository,
the following command does not update a sub repository:

 hg up --clean --rev default

this is a problem especially when using Jenkins CI as it is the command that Jenkins uses to get the latest source code.

Workaround:
make some change to the 'super' repository (such as adding a new text file) and commit and push.

The command used by Jenkins should now update the sub repository OK.