Trying to pgpmime sign an email

trace.el

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  mml2015-epg-sign((part (sign . "pgpmime") (tag-location . 214) (contents . "ing\n-- \nJonathan Irving\nhttps://j0ni.ca\n")))
  mml2015-sign((part (sign . "pgpmime") (tag-location . 214) (contents . "ing\n-- \nJonathan Irving\nhttps://j0ni.ca\n")))
  mml-pgpmime-sign-buffer((part (sign . "pgpmime") (tag-location . 214) (contents . "ing\n-- \nJonathan Irving\nhttps://j0ni.ca\n")))
  mml-generate-mime-1((part (sign . "pgpmime") (tag-location . 214) (contents . "ing\n-- \nJonathan Irving\nhttps://j0ni.ca\n")))
  mml-generate-mime()
  message-encode-message-body()
  message-send-mail(nil)
  message-send-via-mail(nil)
  message-send(nil)
  message-send-and-exit(nil)
  funcall-interactively(message-send-and-exit nil)
  call-interactively(message-send-and-exit nil nil)
  command-execute(message-send-and-exit)