--- a/mod_presence_cache/mod_presence_cache.lua	Wed Aug 24 00:36:54 2016 +0200
+++ b/mod_presence_cache/mod_presence_cache.lua	Mon Aug 29 01:06:42 2016 +0200
@@ -79,6 +79,7 @@
 
 	local username = origin.username;
 	local contact_bare = stanza.attr.to;
+	if not contact_bare then return; end -- probe to self
 
 	local bare_cache_key = username .. "\0" .. contact_bare;