mu4e view as pdf is not working

[nanounanue]

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-executable-p(nil)
  mu4e-action-view-as-pdf((:docid 248047 :subject "Your Droplet's password has been reset. (discourse..." :date (23954 21587 0) :size 5706 :message-id "30039865.20190930191531.5d92545315e406.00945117@ma..." :path "/home/nanounanue/Maildir/nanounanue-gmail/INBOX/cu..." :maildir "/nanounanue-gmail/INBOX" :priority normal :flags (seen) :parts ((:index 1 :name "1.msgpart" :mime-type "text/plain" :type (leaf) :attachment nil :size 249)) :from (("DigitalOcean" . "[email protected]")) :to ((nil . "[email protected]")) :body-txt-params (("charset" . "utf-8")) :body-txt "Your Droplet's password has been reset to: dec8639..."))
  mu4e-view-action()
  funcall-interactively(mu4e-view-action)
  call-interactively(mu4e-view-action nil nil)
  command-execute(mu4e-view-action)

[lapp0]

hi @nanounanue, any luck figuring out the cause of this issue?