(compose-mail): Give a better error message for `mail-user-agent'

* lisp/simple.el (compose-mail): Give a better error message for
invalid values for `mail-user-agent' (bug#17979).

diff --git a/lisp/simple.el b/lisp/simple.el
index dada65d4ee387d42441f9da912c5e7de93390b47..7d94b64913f2e304a5c9266df7f1d2f141f3a8da 100644 (file)
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -7895,6 +7895,8 @@ To disable this warning, set `compose-mail-user-agent-warnings' to nil."
                                               warn-vars " "))))))
 
   (let ((function (get mail-user-agent 'composefunc)))
+    (unless function
+      (error "Invalid value for `mail-user-agent'"))
     (funcall function to subject other-headers continue switch-function
             yank-action send-actions return-action)))