Oh I see - these variables are supposed to be iterables (i.e. things that contain a number of other things that you can iterate over) (i.e. lists or tuples).

@stefanor is right, you probably want a tuple containing the string `'pub_date'`, which looks like:

`('pub_date', )`

Note the comma - this makes the values inside the brackets a comma-separated list, as opposed to simply a string-inside-brackets (which is the same as just-a-string).  If you want a tuple with one element, you need that comma.