--- 11calendar.xml.orig	2014-02-01 22:34:40.268617131 +0100
+++ 11calendar.xml	2014-02-01 22:34:29.664736610 +0100
@@ -261,6 +261,8 @@
     ]]></macro>
 
     <macro name="CALENDAR_BEFOREWRITE_SCRIPT"><![CDATA[
+      if (!UID) { UID = newuid(); }
+
       // set UTC time of generation for iCalendar 2.0 DTSTAMP
       DGENERATED = NOW();
       // ensure that there is a DMODIFIED = LAST-MODIFIED property,