Try to apply json-pretty-print to follow json:

{"t": 1, "key":2}

fail with the message:  json-pretty-print: Bad JSON object key: t

Some research point me the problem is in the function json-encode-key,
its  implementation is:

(defun json-encode-key (object)
  "Return a JSON representation of OBJECT.
If the resulting JSON object isn't a valid JSON object key,
this signals `json-key-format'."
  (let ((encoded (json-encode object)))
    (unless (stringp (json-read-from-string encoded))
      (signal 'json-key-format (list object)))
    encoded))


The follow change seems to  solve the problem, but I haven't had time
understand it in depth:

(defun json-encode-key (object)
   "Return s string representation of a key"
   (json-encode-string (symbol-name object)))