I'm getting this error when running a trigger script which sorts records. I'm using the starter file 'Invoices" as a basis and trying to incorporate some of the functionality into my solution. Script as per the screen shot - only having issues with "Category". DB screen shot also attached. I was thinking it may be something to do with field type i'm trying to sort but I'm using the same settings as the Starter solution.
Any help much appreciated - can't find much about this error online.