I ran into this error when attempting to apply the tablesorter plugin to an empty table, also with multiple tables on the page. In my case, I was able to work around this error by testing for the existence of rows in the table before deciding whether or not to apply tablesorter using jQuery's :has content filter http://docs.jquery.com/Selectors/has#selector
$('#my_table_id:has(tbody tr)').tablesorter({ widgetZebra: { css: ['custom_even', 'odd'] }, sortList: [ [1, 1] ], }); gf