diff --git a/clash/parse-date.lisp b/clash/parse-date.lisp
index bb76372..4ca31ac 100644
--- a/clash/parse-date.lisp
+++ b/clash/parse-date.lisp
@@ -200,7 +200,10 @@ least one range given will match (that is, the union of all given dates).~%"))
                            when (and s e)
                              collect (subseq string s e)
                            else
-                             collect nil))))))))
+                             collect nil)))))))
+  ;; we call return-from above, so this will only run if all know patterns fail
+  ;; to match
+  (error "Invalid date string: ~S" string))
 
 (defun parse-date-range (string)
   "Parse a date range from STRING."