Got a message from a non-member that was stuck in mailman, and I don't know how to retrieve my lost list administrator password. So I just yank it below: Hey Everyone, I have noticed a bit of weird behavior from the json package and was wondering if someone can help me by pointing out what is going on and more importantly why this behavior. - In the below you can see that decode the json string and encoding it results in the output equal to the input as is expected: CL-USER> (json:ENCODE-JSON-TO-STRING (json:decode-json-from-string "{\"available\":true}")) "{\"available\":true}" -Now the issue appears when one attempts to encode the false value as can be seen from the below. As you can see the problem from the second line the real issue is in the encoding of the nil value. CL-USER> (json:ENCODE-JSON-TO-STRING (json:decode-json-from-string "{\"available\":false}")) "[[\"available\"]]" CL-USER> (json:ENCODE-JSON-TO-STRING `((:available . nil))) "[[\"available\"]]" Can anyone point out how to fix this especially that the docs point out that nil will be transformed to null. Thanks to any help you can provide. Regards, Mackram Raydan And my answer is: cl-json has three encoders, for historical reasons the default is the guessing encoder. It has difficulties correctly guessing if nil is false or an empty list. If you want more control I suggest using the explicit encoder. Here are some samples, there are more examples in testencoder.lisp, in particular check the explicit-encoder-complex-objects test. This is in the darcs version, which is the latest version. Hope this helps, Henrik JSON> (json:decode-json-from-string "{\"available\":true}") ((:AVAILABLE . T)) JSON> (json:encode-json-to-string '((:AVAILABLE . T))) "{\"available\":true}" JSON> (json:encode-json-to-string '((:AVAILABLE . nil))) "[[\"available\"]]" JSON> (with-guessing-encoder (json:encode-json-to-string '((:AVAILABLE . nil)))) "[[\"available\"]]" (you see guessing encode is default) JSON> (with-explicit-encoder (json:encode-json-to-string '(:object (:avaliable . nil)))) "{\"avaliable\":null}" JSON> (with-explicit-encoder (json:encode-json-to-string '(:object (:avaliable . :false)))) "{\"avaliable\":\"false\"}" JSON> (with-explicit-encoder (json:encode-json-to-string '(:object (:avaliable . :true)))) "{\"avaliable\":\"true\"}" JSON> (with-explicit-encoder (encode-json-to-string `(:object :available ,(json-bool (if t t nil))))) "{\"available\":true}" JSON> (with-explicit-encoder (encode-json-to-string `(:object :available ,(json-bool (if nil t nil))))) "{\"available\":false}"