Can't prune inactive users

[Snowhog]

vBulletin 4.2.0

I'm attempting to prune (delete) inactive members in our forum.

Admin CP > Users > Prune / Move Users

with the following settings:

All usergroups
Has not logged in for 365 days
Join Date is Before August 1, 2012 (I've even left this 'blank')
Posts is less than 1
Order By Name

This results with: Showing Users 1 to 6750 of 6750, and they are all 'checked'. When I select Action: Delete and click Go, vBulletin reports:

Invalid Action Specified followed by No Users Matched your Query

What gives? Why is vBulletin identifying the desired inactive users, but then says I've selected an invalid action and that no users matched my query, when obviously, it did!?

Added: Got it figured out. Apparently there is a limit to the number of records that can be marked for deletion at one time, which seems to be 1,000. I have been able to delete the 6,750 inactive users.