--- a/plugins/muc/muc.lib.lua	Thu May 17 17:12:07 2018 +0200
+++ b/plugins/muc/muc.lib.lua	Tue May 29 22:33:22 2018 +0200
@@ -365,7 +365,7 @@
         return self._data.default_history_messages or default_history_length;
 end
 function room_mt:set_defaulthistorymessages(number)
-        number = math.min(tonumber(number) or default_history_length, self._data.history_length);
+        number = math.min(tonumber(number) or default_history_length, self._data.history_length or default_history_length);
         if number == default_history_length then
                 number = nil;
         end