diff --git a/src/Expr.h b/src/Expr.h
index f422fc0df0..05771d956a 100644
--- a/src/Expr.h
+++ b/src/Expr.h
@@ -1302,7 +1302,7 @@ public:
 	LambdaExpr(std::unique_ptr<function_ingredients> ingredients,
 	           IDPList outer_ids);
 
-	IDPList OuterIDs() const	{ return outer_ids; }
+	const IDPList& OuterIDs() const	{ return outer_ids; }
 
 	ValPtr Eval(Frame* f) const override;
 	TraversalCode Traverse(TraversalCallback* cb) const override;
diff --git a/src/script_opt/GenRDs.cc b/src/script_opt/GenRDs.cc
index 2645323b08..5979be09ba 100644
--- a/src/script_opt/GenRDs.cc
+++ b/src/script_opt/GenRDs.cc
@@ -1126,7 +1126,7 @@ TraversalCode RD_Decorate::PreExpr(const Expr* e)
 	case EXPR_LAMBDA:
 		{
 		auto l = dynamic_cast<const LambdaExpr*>(e);
-		auto ids = l->OuterIDs();
+		const auto& ids = l->OuterIDs();
 
 		for ( auto& id : ids )
 			CheckVar(e, id, false);