When I click on an internal link to an anchor, I get the error message:

"Wrong type argument: number-or-marker-p"

The same happens if I type C-c C-o on the link.  This is mapped
to org-open-at-point, which should handle internal links.

However, if I type C-c o on the link, then the cursor goes to the anchor.
This is mapped to org-open-at-point-global, which the org info help says is
undefined on internal links.

This behavior is new, I believe since the last org package update.  I'm
currently on org mode version 9.1.2.

------ Example Org File -----------

* headline 1
  <<Anchor 1>>
* headline 2
*** [[Anchor 1]]  CLICK ON THIS