[plt-scheme] continuation question

Hi all,

I'm trying to better understand continuations.  When this code runs  
at the top level, it does not loop, unless I use the the let instead  
of the begin.  Why?  Is the call to "k" failing? If so, how come it  
doesn't throw an error?

(define k #f)
;(let ()
(begin
   (let/cc c (set! k c))
   (printf "calling continuation...\n")
   (k 'whatever)
   (printf "done.\n"))


thanks,
Rob