struct #:mutable field option

[bennn]

What version of Racket are you using?

$ racket --version
Welcome to Racket v8.9.0.3 [cs].

What program did you run?

;#lang racket
;(struct node (val [next #:mutable]))

#lang typed/racket
(struct node ([val : Real] [next : node #:mutable]))

What should have happened?

No error

If you got an error message, please include it here.

struct: unexpected term
  at: #:mutable
  in: (struct node ((val : Real) (next : node #:mutable)))
  parsing context: 
   while parsing [field-name : type]
    term: (next : node #:mutable)
    location: /home/ben/foo.rkt:5:27