This patch accompanies the HEAD patch:

  Tue Sep 14 12:38:27 BST 2010  simonpj@microsoft.com
    * Make absent-arg wrappers work for unlifted types (fix Trac #4306)
    
    Previously we were simply passing arguments of unlifted
    type to a wrapper, even if they were absent, which was
    stupid.
    
    See Note [Absent error Id] in WwLib.