Bad value to a defcustom makes choices unavailable

[Lennart Borgman]

If the value of a defcustom with ":type '(choices ...)" has a bad value 
then the choice menu is unavailable. Try the following for example:

(defcustom xhtml-help-refurl "ttp://www.site1.com/"
 "Web url to get references from."
 :type '(choice (const "http://www.site1.com/";)
                (const "http://www.site2.com//";)
                ))