sqlite 3.4 updating table with data from different table - changed with 3.3 - newbie question

All of the other questions on this seem to be before 3.3 when sqlite added an update join feature? Newbie here, so I may even be getting the terminology wrong. I am trying to update the issbn field in table GULLfromLG (currently empty), with data from the issbn field in BIBinfo (field can be empty in a given record). I am using SQLiteStudio. From looking through the documentation, sqlitetutorial, w3resource, and general web searches I believe the following should work. But it doesn't. The processing happens, is declared successful, but all the fields in GULLfromLG are empty. What am I missing? UPDATE GULLfromLG SET issbn = ( select issbn FROM BIBinfo WHERE bibno = bibno) I have also tried naming the fields with variations of `GULLfromLG.issbn GULLfromLG.bibno BIBinfo.issbn BIBinfo.bibno` Any help would be greatly appreciated!