[racket] Problem with structs and #lang racket/signature

There seems to be a problem exporting struct constructors when using #lang
racket/signature. This works:

----- b-sig.rkt-----
#lang racket

(define-signature b^
  ((struct my-struct (a b c))
   b-value))

(provide b^)

----- b-unit.rkt -----
#lang racket/unit

(require "b-sig.rkt")

(import)
(export b^)

(struct spelling-word
  (word sentence word-number lesson word-list))

(define b-value 3)

But if you change b-sig to use #lang racket/signature:
#lang racket/signature

(struct my-struct (a b c))
b-value

then running b-unit produces the error:

Welcome to DrRacket, version 6.1 [3m].
Language: racket/unit; memory limit: 512 MB.
define-unit: undefined export make-my-struct in: (define-unit b@ (import)
(export b^) (struct my-struct (a b c)) (define b-value 3))

Is this a just a bug or am I missing something?
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