When using display tag in external sorting/paging mode you have to specify the size parameter. I've done that, but I'm getting the above error.
The instructions for doing this are here: http://www.displaytag.org/1.2/tut_externalSortAndPage.html
A lot of other people have had this problem too and I haven't seen a good solution yet. Typically the solution is to pass the parameter using a scriptlet. I would prefer to do this using an expression.
Here is my code:
<display:table name="hitlistModel.hitResults" id="hitItem" class="HitlistResults" defaultorder="ascending"
sort="external" size="${hitlistModel.totalCountRecords}" partialList="true" pagesize="${hitlistModel.pageSize}" cellspacing="1" cellpadding="0"
requestURI="Hitlist.htm" defaultsort="2" export="true">