1 files changed, 6 insertions, 0 deletions

diff --git a/lisp/calendar/solar.el b/lisp/calendar/solar.el
index 57a6c6a40a8..f8e747a317c 100644
--- a/lisp/calendar/solar.el
+++ b/lisp/calendar/solar.el
@@ -4,6 +4,7 @@
 
 ;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
 ;;      Denis B. Roegel <Denis.Roegel@loria.fr>
+;; Maintainer: Glenn Morris <gmorris@ast.cam.ac.uk>
 ;; Keywords: calendar
 ;; Human-Keywords: sunrise, sunset, equinox, solstice, calendar, diary,
 ;;      holidays
@@ -207,6 +208,11 @@ Needed for polar areas, in order to know whether the day lasts 0 or 24 hours.")
 
 (defun solar-setup ()
   "Prompt user for latitude, longitude, and time zone."
+  (unless (or (and calendar-longitude calendar-latitude calendar-time-zone)
+              (interactive-p))
+    ;; Defaults must be nil, else results could be subtlely wrong for
+    ;; user's real location.
+    (error "Calendar latitude, longitude and/or time-zone unset"))
   (beep)
   (if (not calendar-longitude)
       (setq calendar-longitude