typing callcc

Perhaps this is obvious or a FAQ, but why does the following example (from
Harper, Duba, and MacQueen) result in a type error?

callcc(fn k => if true then [ (throw k 3) + 1 ] else 5 :: (throw k 4))

stdIn:2.1-2.71 Error: operator and operand don't agree [literal]
  operator domain: int cont -> int
  operand:         int cont -> int list
  in expression:
    callcc (fn k => if true then <exp> :: <exp> else <exp> :: <exp>)



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