Jquery plugin tablesorter: sorting stops after a question mark has made it to the top row

I have this column: D, B, A, ? On clicking the column header this sorts to: ?,A,B,D. Bu开发者_如何学运维t after that clicking the column header does nothing. If I replace the question mark with say Z, it all starts working agian

That is very odd... but, I found that if you set the sorter to "text", it fixes this issue.

$('table').tablesorter({
    headers : {
        0: { sorter: "text" }
    } 
});

Here is a demo. In the numeric column, the "?" is treated like a zero.