Nested unit fails with "missing type for `check-unit`"

What version of Racket are you using?

v6.9.0.1

What program did you run?

#lang typed/racket/base

(require typed/racket/unit)

(define-unit a@
  (import)
  (export)

  (define-unit b@
    (import)
    (export))

  (define-values/invoke-unit/infer b@))

What should have happened?

Program ok

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

; Type Checker: missing type for identifier;
;  consider using `require/typed' to import it
;   identifier: check-unit
;   from module: private/unit-runtime.rkt
;   in: b@