diff --git a/xo-reader/include/xo/reader/expect_formal_arglist_xs.hpp b/xo-reader/include/xo/reader/expect_formal_arglist_xs.hpp
index f3a6bf22..ee3a7a48 100644
--- a/xo-reader/include/xo/reader/expect_formal_arglist_xs.hpp
+++ b/xo-reader/include/xo/reader/expect_formal_arglist_xs.hpp
@@ -16,7 +16,7 @@ namespace xo {
          *  ^ ^                 ^ ^    ^
          *  | |                 | |    |
          *  | |                 | |    argl_1b
-         *  | argl_1a           | argla
+         *  | argl_1a           | argl_1a
          *  argl_0              argl_1b
          *
          *  argl_0 --on_leftparen_token()--> argl_1a
@@ -55,6 +55,8 @@ namespace xo {
 
             static void start(parserstatemachine * p_psm);
 
+            virtual const char * get_expect_str() const override;
+
             virtual void on_leftparen_token(const token_type & tk,
                                             parserstatemachine * p_psm) override;
             virtual void on_formal(const rp<Variable> & formal,
diff --git a/xo-reader/src/reader/expect_formal_arglist_xs.cpp b/xo-reader/src/reader/expect_formal_arglist_xs.cpp
index 916841da..f05680d9 100644
--- a/xo-reader/src/reader/expect_formal_arglist_xs.cpp
+++ b/xo-reader/src/reader/expect_formal_arglist_xs.cpp
@@ -43,6 +43,24 @@ namespace xo {
             p_psm->push_exprstate(expect_formal_arglist_xs::make());
         }
 
+        const char *
+        expect_formal_arglist_xs::get_expect_str() const {
+            switch (farglxs_type_) {
+            case formalarglstatetype::invalid:
+            case formalarglstatetype::n_formalarglstatetype:
+                assert(false); // impossible
+                break;
+            case formalarglstatetype::argl_0:
+                return "leftparen";
+            case formalarglstatetype::argl_1a:
+                return "formal-name";
+            case formalarglstatetype::argl_1b:
+                return "comma|rightparen";
+            }
+
+            return "?expect";
+        }
+
         expect_formal_arglist_xs::expect_formal_arglist_xs()
             : exprstate(exprstatetype::expect_formal_arglist),
               farglxs_type_{formalarglstatetype::argl_0}