make index fail

Whenever I update my ports tree (git pull) I then do a make index.
Over the last week or so it has been failing like this:

%%%
root@mater:/usr/ports # make index
Generating INDEX-13 - please wait..--- describe.accessibility --- [0]
--- describe.arabic ---
  ...
--- describe.x11-wm ---
make_index: /usr/ports/accessibility/kmouth: no entry for 
/usr/ports/accessibility/qt5-speech
  Done.
%%%

After this /usr/ports/INDEX-13 is 0 bytes.

Any hints as to how to fix?

[0] have always wondered why there is no newline after the 'please wait..'

-andyf