Bug 27316 removed the ability to have "Undef" as a sorting option. It is now either yes (1) or no (0). The default mappings.yaml file still contains a lot of sort: ~, which I assume is "Undef". I propose to change the ~ to 0 so that the mappings file is clean.
It looks like this was fixed with the first patch of: Bug 33594 - Sorting results by Title A-Z might use wrong title field