One of my colleagues has started using S/MIME to sign messages.
But when I try to read them, gnus (5.10.7) goes bang:

Debugger entered--Lisp error: (error "Invalid argument 3")
  call-process-region(1 7396 nil nil ("*OpenSSL output*" t) nil "smime" "-verify" "-noverify" "-out" "/dev/null")
  apply(call-process-region 1 7396 nil nil ("*OpenSSL output*" t) nil ("smime" "-verify" "-noverify" "-out" "/dev/null"))
  smime-call-openssl-region(1 7396 ("*OpenSSL output*" t) "smime" "-verify" "-noverify" "-out" "/dev/null")
  smime-noverify-region(1 7396)
  smime-noverify-buffer()
  mml-smime-verify(((#("multipart/mixed" 0 15 ...) (#<buffer  *mm*<13>> ... 7bit nil nil nil nil nil) (#<buffer  *mm*<14>> ... 7bit nil ... nil nil nil)) (#<buffer  *mm*<15>> ("application/x-pkcs7-signature" ...) base64 nil ("attachment" ...) "S/MIME Cryptographic Signature" nil nil)) (#("multipart/signed" 0 16 (start nil from "colleague@host.com" buffer #<buffer  *mm*<11>> boundary "------------ms070502010904030200030703" micalg "sha1" protocol "application/x-pkcs7-signature")) (protocol . "application/x-pkcs7-signature") (micalg . "sha1") (boundary . "------------ms070502010904030200030703")))
  mm-possibly-verify-or-decrypt(((#("multipart/mixed" 0 15 ...) (#<buffer  *mm*<13>> ... 7bit nil nil nil nil nil) (#<buffer  *mm*<14>> ... 7bit nil ... nil nil nil)) (#<buffer  *mm*<15>> ("application/x-pkcs7-signature" ...) base64 nil ("attachment" ...) "S/MIME Cryptographic Signature" nil nil)) (#("multipart/signed" 0 16 (start nil from "colleague@host.com" buffer #<buffer  *mm*<11>> boundary "------------ms070502010904030200030703" micalg "sha1" protocol "application/x-pkcs7-signature")) (protocol . "application/x-pkcs7-signature") (micalg . "sha1") (boundary . "------------ms070502010904030200030703")))
  mm-dissect-multipart((#("multipart/signed" 0 16 (start nil from "colleague@host.com" buffer #<buffer  *mm*<11>> boundary "------------ms070502010904030200030703" micalg "sha1" protocol "application/x-pkcs7-signature")) (protocol . "application/x-pkcs7-signature") (micalg . "sha1") (boundary . "------------ms070502010904030200030703")) "colleague@host.com")
  mm-dissect-buffer(nil nil)
  gnus-display-mime()
  gnus-article-prepare-display()
  gnus-article-prepare(8170 nil)
  gnus-summary-display-article(8170)
  gnus-summary-next-page(nil)
* call-interactively(gnus-summary-next-page)

My best guess is that smime.el is trying to run a program called "smime".
What is that and where do I get it? Or how do I make this error go away,
in any event?

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | 'Tis better to be silent and be thought
http://nwalsh.com/            | a fool, than to speak and remove all
                              | doubt.--Abraham Lincoln