Behave better when #'variable-at-point gives nil, #'custom-variable-prompt.
lisp/ChangeLog addition:
2009-09-27 Aidan Kehoe <kehoea@parhasard.net>
* cus-edit.el (custom-variable-prompt):
nil is a symbol, check that variable-at-point is non-nil before
checking if it's a symbol when deciding on the prompt used in this
function.