diff --git a/include/xo/reader2/ParserStateMachine.hpp b/include/xo/reader2/ParserStateMachine.hpp
index fb276bd0..b333dd51 100644
--- a/include/xo/reader2/ParserStateMachine.hpp
+++ b/include/xo/reader2/ParserStateMachine.hpp
@@ -69,6 +69,10 @@ namespace xo {
             /** pop syntax state machine from top of @ref stack_ **/
             void pop_ssm();
 
+            /** get unique string copy of @p str. Idempotent for each @p str.
+             **/
+            const DUniqueString * intern_string(std::string_view str);
+
             /** add variable to current local environment (innermost lexical scope) **/
             void upsert_var(DVariable * var);
 
diff --git a/src/reader2/DDefineSsm.cpp b/src/reader2/DDefineSsm.cpp
index adf9f429..eabc1868 100644
--- a/src/reader2/DDefineSsm.cpp
+++ b/src/reader2/DDefineSsm.cpp
@@ -412,12 +412,15 @@ namespace xo {
         }
 
         void
-        DDefineSsm::on_parsed_symbol(std::string_view sym,
+        DDefineSsm::on_parsed_symbol(std::string_view sym_name,
                                      ParserStateMachine * p_psm)
         {
             if (this->defstate_ == defexprstatetype::def_1) {
                 this->defstate_ = defexprstatetype::def_2;
 
+                const DUniqueString * sym
+                    = p_psm->intern_string(sym_name);
+
                 def_expr_.data()->assign_lhs_name(sym);
 
                 // if this is a genuine top-level define (i.e. nesting level = 0),
@@ -448,7 +451,7 @@ namespace xo {
             }
 
             p_psm->illegal_input_on_symbol("DDefineSsm::on_parsed_symbol",
-                                           sym,
+                                           sym_name,
                                            this->get_expect_str());
         }
 
diff --git a/src/reader2/ParserStateMachine.cpp b/src/reader2/ParserStateMachine.cpp
index 1766f652..ca8bba8b 100644
--- a/src/reader2/ParserStateMachine.cpp
+++ b/src/reader2/ParserStateMachine.cpp
@@ -73,6 +73,12 @@ namespace xo {
             this->stack_ = ParserStack::pop(stack_, parser_alloc_);
         }
 
+        const DUniqueString *
+        ParserStateMachine::intern_string(std::string_view str)
+        {
+            return stringtable_.intern(str);
+        }
+
         void
         ParserStateMachine::upsert_var(DVariable * var)
         {