--- a/plugins/muc/muc.lib.lua	Tue Sep 04 18:00:43 2018 +0200
+++ b/plugins/muc/muc.lib.lua	Tue Sep 04 21:55:40 2018 +0100
@@ -1465,7 +1465,7 @@
 	local occupants = {};
 	local room_name, room_host = jid_split(room_jid);
 
-	room._affiliation_data = frozen._affiliation_data;
+	room._affiliation_data = frozen._affiliation_data or {};
 
 	if frozen.jid and frozen._affiliations then
 		-- Old storage format