views with 2 taxonomy filters via a relationship hangs - problem in views_many_to_one_helper

I have a two taxonomy term ref fields on users, and a view with the following:

- base: user
- relationship 1: first taxonomy field, multi-valued
- relationship 2: second taxo field, multi-valued
- filter 1: taxonomy on the first relationship, single-valued
- filter 2: taxonomy on the second relationship, single-valued

If I pick an option from just one filter, then everything works fine.
If I pick an option from each filter, the page load hangs.

Here's how far I've got with debugging this...

The site log has this:

Notice: Trying to get property of non-object in views_many_to_one_helper->add_table() (line 710 of /Users/joachim/Sites/rbs-drupal-p3/sites/all/modules/views/includes/handlers.inc).

This is within the views_many_to_one_helper object, in add_table(). It's trying to work with joins but it doesn't have what it needs.

I think I've traced the problem further back. In views_many_to_one_helper->ensure_my_table(), we have this logic:

        if (empty($this->handler->options['add_table']) && empty($this->handler->view->many_to_one_tables[$field])) {

For the first filter, this check passes because the many_to_one_tables array is empty.
The second time we come here, the value of $field is the same. I can't tell if that's part of the bug too, or if that's normal!

At any rate, having failed that condition, we go to the else part and the first statement there is:

          $join = $this->get_join();

which returns NULL. Again, I suspect this is a bug, but the code in get_join() appears to allow for the possibility of returning NULL so I am not sure. What I do know is that from here, we have a $join object that doesn't start out with a full set of keys, and so that is that I think causes the error message in add_table(), which is called further along.