--- a/util/set.lua	Sun Jan 22 23:57:13 2012 +0000
+++ b/util/set.lua	Sun Jan 22 23:59:19 2012 +0000
@@ -82,8 +82,10 @@
 	end
 	
 	function set:add_list(list)
-		for _, item in ipairs(list) do
-			items[item] = true;
+		if list then
+			for _, item in ipairs(list) do
+				items[item] = true;
+			end
 		end
 	end