--- a/mod_mam/mod_mam.lua	Sun Sep 03 00:35:31 2017 +0200
+++ b/mod_mam/mod_mam.lua	Sun Sep 03 12:43:24 2017 +0200
@@ -330,6 +330,7 @@
 
 local function strip_stanza_id(event)
 	local strip_by = jid_bare(event.origin.full_jid);
+	event.stanza = st.clone(event.stanza);
 	event.stanza:maptags(function(tag)
 		if not ( tag.attr.xmlns == xmlns_st_id and tag.attr.by == strip_by ) then
 			return tag;