When ByWater started upgrading our partners to 17.11 last spring, I noticed that we were getting a lot of errors of the form "can't call method * on an undefined value at ..." We're starting to see these errors because more and more of our code uses Koha objects, and these will fail if a method is called on an undefined object. I brought this up here: http://lists.koha-community.org/pipermail/koha-devel/2018-June/044613.html I think that there was also another thread about this, which I can't find at the moment... Jonathan's opinion on this is that we should start creating queries which find data that will fail. This thread is for those queries, since I have an example: Circulating or holdable items without item type SELECT biblionumber, itemnumber, barcode, title, notforloan FROM items INNER JOIN biblio USING (biblionumber) WHERE notforloan < 1 AND itype IS NULL Once we have a few of these at hand, I'll start a page on the Koha wiki.