diff --git a/lisp/gnus/mm-uu.el b/lisp/gnus/mm-uu.el
index 9d4c4bfead7..647b97c8662 100644
--- a/lisp/gnus/mm-uu.el
+++ b/lisp/gnus/mm-uu.el
@@ -511,7 +511,7 @@ apply the face `mm-uu-extract'."
       (list (mm-make-handle buf mm-uu-text-plain-type)))))
 
 (defun mm-uu-pgp-signed-extract ()
-  (let ((mm-security-handle (list (format "multipart/signed"))))
+  (let ((mm-security-handle (list (substring "multipart/signed"))))
     (mm-set-handle-multipart-parameter
      mm-security-handle 'protocol "application/x-gnus-pgp-signature")
     (save-restriction