diff --git a/include/xo/reader/expect_formal_arglist_xs.hpp b/include/xo/reader/expect_formal_arglist_xs.hpp
index 82116133..442dde62 100644
--- a/include/xo/reader/expect_formal_arglist_xs.hpp
+++ b/include/xo/reader/expect_formal_arglist_xs.hpp
@@ -53,7 +53,7 @@ namespace xo {
         public:
             expect_formal_arglist_xs();
 
-            static std::unique_ptr<expect_formal_arglist_xs> make();
+            static void start(exprstatestack * p_stack);
 
             virtual void on_leftparen_token(const token_type & tk,
                                             exprstatestack * p_stack,
@@ -69,6 +69,9 @@ namespace xo {
                                              rp<Expression> * p_emit_expr) override;
             virtual void print(std::ostream & os) const override;
 
+        private:
+            static std::unique_ptr<expect_formal_arglist_xs> make();
+
         private:
             /** parsing state-machine state **/
             formalarglstatetype farglxs_type_ = formalarglstatetype::argl_0;
diff --git a/src/reader/expect_formal_arglist_xs.cpp b/src/reader/expect_formal_arglist_xs.cpp
index 2c67c474..6f2a4e13 100644
--- a/src/reader/expect_formal_arglist_xs.cpp
+++ b/src/reader/expect_formal_arglist_xs.cpp
@@ -35,6 +35,12 @@ namespace xo {
                 (expect_formal_arglist_xs());
         }
 
+        void
+        expect_formal_arglist_xs::start(exprstatestack * p_stack)
+        {
+            p_stack->push_exprstate(expect_formal_arglist_xs::make());
+        }
+
         expect_formal_arglist_xs::expect_formal_arglist_xs()
             : exprstate(exprstatetype::expect_formal_arglist),
               farglxs_type_{formalarglstatetype::argl_0}
diff --git a/src/reader/lambda_xs.cpp b/src/reader/lambda_xs.cpp
index 82b91ae2..c9c131ac 100644
--- a/src/reader/lambda_xs.cpp
+++ b/src/reader/lambda_xs.cpp
@@ -23,7 +23,7 @@ namespace xo {
         {
             if (lmxs_type_ == lambdastatetype::lm_0) {
                 this->lmxs_type_ = lambdastatetype::lm_1;
-                p_stack->push_exprstate(expect_formal_arglist_xs::make());
+                expect_formal_arglist_xs::start(p_stack);
             } else {
                 exprstate::on_lambda_token(tk, p_stack, p_emit_expr);
             }