Case expressions in STG

I am reading SPJ's seminal work "Implementing lazy functional languages on stock hardware: the Spinless Tagless G-machine" (1992). The paper is available here http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.3729 On p62 we see "The expression scrutinised by a case expression must eventually evaluate either to a primitive value or a constructor application.". This doesn't make sense to me. Isn't the following a valid STG program? What am I missing? Where is it specified that the scrutinee of a case expression cannot be of a function type? {x} \n {z} -> let f = \{x} \n {y} -> g x y in case f of f' -> f' z Thanks, Tom