--- a/plugins/mod_groups.lua	Sat Jul 07 21:00:14 2018 +0200
+++ b/plugins/mod_groups.lua	Sat Jul 07 21:00:59 2018 +0200
@@ -112,6 +112,8 @@
 				groups[curr_group][jid] = name or false;
 				members[jid] = members[jid] or {};
 				members[jid][#members[jid]+1] = curr_group;
+			elseif entryjid:match("%S") then
+				module:log("warn", "Invalid JID: %q", entryjid);
 			end
 		end
 	end