Invalid Argument in Album.php

I get a "Invalid argument in foreach in .../Album.php line 2242" after clicking on a thumbnail in Gallery 1.5x. It doesn't appear for thumbnail views, just for the intermediate photo views and the full-size views.

It doesn't happen to every picture and it doesn't effect anything other then messing up the display by adding the error message to top of the webpage. I've have run the upgrade_album.php, several times. All it does is to appear with a done button, that doesn't do anything.

This message also occurs to new pictures in new subalbums (but not in new main albums) in old main albums, created after the upgrade.. I tried commenting out the code, but new errors appear elsewhere downstream.