maybe a bug in typed racket?

typed-racket-bug.rkt

#lang typed/racket/base

;; These are the primitives that we know how to inline.
(define-type KernelPrimitiveName/Inline (U '+
                                           '-
                                           '*
                                           '/
                                           'zero?
                                           'add1
                                           'sub1
                                           '<
                                           '<=
                                           '=
                                           '>
                                           '>=
                                           'cons
                                           'car
                                           'caar
                                           'cdr
                                           'list
                                           'list?
                                           'pair?
                                           'null?
                                           'not
                                           'eq?
                                           'vector-ref
                                           'vector-set!
                                           ))

(: operator->string (KernelPrimitiveName/Inline -> String))
(define (operator->string operator)
  (case operator
          [(+)
           "+"]
          
          [(-)
           "-"]
          
          [(*)
           "*"]

          [(/)
           "/"]

          [(zero?)
           "zero?"]

          [(add1)
           "add1"]
          
          [(sub1)
           "sub1"]

          [(<)
           "<"]

          [(<=)
           "<="]
          
          [(=)
           "="]
          
          [(>)
           ">"]
          
          [(>=)
           ">="]
          
          [(cons)
           "cons"]

          [(car)
           "car"]

          [(caar)
           "caar"]
          
          [(cdr)
           "cdr"]
          
          [(list)
           "list"]

          [(list?)
           "list?"]
          
          [(pair?)
           "pair?"]
          
          [(null?)
           "null?"]

          [(not)
           "not"]
          
          [(eq?)
           "eq?"]
          
          [(vector-ref)
           "vector-ref"]
          
          [(vector-set!)
           "vector-set!"]))