Thanks for the awesome work.
I was wondering if it's possible to somehow allow nullable fields in the define-json-type macro.
I think that this is indeed the expected behavior.

Example

(define-json-type <student>
  (id)
  (name)
  (major "major" <major>))

(define-json-type <major>
  (id)
  (name)
  (parent "parent" <major>))

(define json-str "{ \"id\": 1, \"name\": null, \"major\": null}")
(json->student json-str)

error

In procedure assoc: Wrong type argument in position 2 (expecting association list): null

changing the json-str to the below

builds a correct (not unspecified) scheme object

(define json-str "{ \"id\": 1, \"name\": null, \"major\": {}}")
(json->student json-str)

output

=> #<<student> id: 1 name: null major: #<<major> id: #<unspecified> name: #<unspecified> parent: #<unspecified>>>

also emitting the field major will produce correct results.

(define json-str "{ \"id\": 1, \"name\": null }")
(json->student json-str)

output

=> #<<student> id: 1 name: null major: #<unspecified>>

Anything wrong with my approach?
Thanks again for your great work.