--- a/plugins/muc/muc.lib.lua	Fri Jan 04 15:13:52 2019 +0100
+++ b/plugins/muc/muc.lib.lua	Sun Jan 06 11:28:54 2019 +0100
@@ -23,6 +23,7 @@
 local st = require "util.stanza";
 local base64 = require "util.encodings".base64;
 local md5 = require "util.hashes".md5;
+local id = require "util.id";
 
 local log = module._log;
 
@@ -1037,6 +1038,9 @@
 function room_mt:handle_groupchat_to_room(origin, stanza)
 	local from = stanza.attr.from;
 	local occupant = self:get_occupant_by_real_jid(from);
+	if not stanza.attr.id then
+		stanza.attr.id = id.medium()
+	end
 	if module:fire_event("muc-occupant-groupchat", {
 		room = self; origin = origin; stanza = stanza; from = from; occupant = occupant;
 	}) then return true; end