isnan is a 515 feature

isnum returning true for nan is a 515 feature...

code/game/objects/items/stacks/stack.dm

@@ -144,15 +144,21 @@ Also change the icon to reflect the amount of sheets, if possible.*/
 		list_recipes(usr, text2num(href_list["sublist"]))
 
 	if(href_list["make"])
-		if(amount < 1) qdel(src) //Never should happen
+		if(amount < 1)
+			qdel(src) //Never should happen
+			return
 
 		var/list/recipes_list = recipes
 		if(href_list["sublist"])
 			var/datum/stack_recipe_list/srl = recipes_list[text2num(href_list["sublist"])]
 			recipes_list = srl.recipes
 		var/datum/stack_recipe/R = recipes_list[text2num(href_list["make"])]
 		var/multiplier = text2num(href_list["multiplier"])
-		if(!isnum(multiplier))
+		if(multiplier != multiplier) // isnan
+			message_admins("[key_name_admin(usr)] has attempted to multiply [src] with NaN")
+			return
+		if(!isnum(multiplier)) // this used to block nan...
+			message_admins("[key_name_admin(usr)] has attempted to multiply [src] with !isnum")
 			return
 		multiplier = round(multiplier)
 		if(multiplier < 1)