mark the json text below and run M-x json-pretty-print

  {"t": "hello"}

Debugger entered--Lisp error: (json-key-format t)
  signal(json-key-format (t))
  json-pretty-print(1 15 nil)
  funcall-interactively(json-pretty-print 1 15 nil)
  call-interactively(json-pretty-print record nil)
  command-execute(json-pretty-print record)
  execute-extended-command(nil "json-pretty-print" "json")
  funcall-interactively(execute-extended-command nil "json-pretty-print"
"json")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)