diff --git a/simple-comment-markup.el b/simple-comment-markup.el
index 9f51da0..6482f6b 100644
--- a/simple-comment-markup.el
+++ b/simple-comment-markup.el
@@ -58,20 +58,25 @@
                  :value-type sexp)))
 
 (defcustom simple-comment-markup-set '(org url)
-  "docstring"
-  :type '(choice symbol (repeat symbol)))
+  "The markup pattern sets to use (see `simple-comment-markup-patterns')."
+  :type '(choice symbol (repeat symbol))
+  :safe (lambda (value)
+          (and (listp value)
+               (null (delq t (mapcar #'symbolp value))))))
 
 (defcustom simple-comment-markup-pre-rx
   (rx (any space ?- ?\( ?' ?\" ?\{))
   "Regexp that must occur immediately before a markup pattern.
 This must not include any capturing groups."
-  :type 'regexp)
+  :type 'regexp
+  :safe #'stringp)
 
 (defcustom simple-comment-markup-post-rx
   (rx (or (any space ?- ?. ?, ?\; ?: ?! ?? ?' ?\" ?\) ?\} ?\\ ?\[) line-end))
   "Regexp that must occur immediately after a markup pattern.
 This must not include any capturing groups."
-  :type 'regexp)
+  :type 'regexp
+  :safe #'stringp)
 
 (defface simple-comment-markup-face
   '((t :inherit shadow))