diff --git a/data/lua/wml-tags.lua b/data/lua/wml-tags.lua
index a91bd24249d..c2497b79304 100644
--- a/data/lua/wml-tags.lua
+++ b/data/lua/wml-tags.lua
@@ -933,7 +933,7 @@ end
 function wml_actions.harm_unit(cfg)
 	local filter = helper.get_child(cfg, "filter") or helper.wml_error("[harm_unit] missing required [filter] tag")
 	-- we need to use shallow_literal field, to avoid raising an error if $this_unit (not yet assigned) is used
-	if not cfg.__shallow_literal.amount then helper.wml_error("[harm_unit] has missing required amount= attribute") end
+	if not helper.shallow_literal(cfg).amount then helper.wml_error("[harm_unit] has missing required amount= attribute") end
 	local variable = cfg.variable -- kept out of the way to avoid problems
 	local _ = wesnoth.textdomain "wesnoth"
 	-- #textdomain wesnoth