(com.inuoe.jzon:stringify
  'cl:null
   :replacer (lambda (k v) (values t "null")))
                                                                 
#|                                                               
Signals the error:

The set of methods
[...]
applicable to arguments #<COM.INUOE.JZON:WRITER {100C31B1F3}>,
"null" to call-next-method is different from the set of methods
[...]
applicable to the original method arguments
#<COM.INUOE.JZON:WRITER {100C31B1F3}>, NULL.
|#