error when using nnfolder

[chylli]

I set nnfolder configure in my emacs as following:

(setq gnus-select-method '(nntp "localhost"))
(setq gnus-secondary-select-methods '((nnfolder "")))
(setq user-full-name "chylli")
(setq sendmail-program "/usr/bin/msmtp")

;; mail from getmail program
(eval-after-load "mail-source"
  '(add-to-list 'mail-sources
                '(maildir :path "~/Maildir/" 
                          :subdirs ("cur" "new"))))


But when I press q to quit that mail after reading mail in
nnfolder:mail.misc, I got an error message:

nnfolder-save-buffer: Wrong type argument: stringp, nil

I can't quit that buffer except use ctrl-x k . 

my mail is fetched by getmail and stored in ~/Maildir as maildir
format.

how should I do to resolve that error ?

when I use nnml instead of nnfolder, that error disappeared.


-- 

Thanks & Regards

Chylli