diff --git a/include/picrin/value.h b/include/picrin/value.h
index 6fa2b75d..ef763a0a 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -9,6 +9,13 @@
 extern "C" {
 #endif
 
+/**
+ * pic_sym is just an alias to int.
+ * the value 0 for pic_sym is guaranteed to resolve to no symbol.
+ * if you are defining a function that returns optional<pic_sym>,
+ * the zero symbol would be useful for such situation.
+ */
+
 typedef int pic_sym;
 
 /**
diff --git a/src/state.c b/src/state.c
index 4875193e..f647af15 100644
--- a/src/state.c
+++ b/src/state.c
@@ -75,6 +75,9 @@ pic_open(int argc, char *argv[], char **envp)
   /* native stack marker */
   pic->native_stack_start = &t;
 
+  /* symbol 0 is reserved for system */
+  xh_put_int(pic->sym_names, pic->sym_cnt++, (long)"<system-reserved-symbol>");
+
 #define register_core_symbol(pic,slot,name) do {	\
     pic->slot = pic_intern_cstr(pic, name);		\
   } while (0)
@@ -155,6 +158,8 @@ pic_close(pic_state *pic)
 
   /* free symbol names */
   for (xh_begin(pic->sym_names, &it); ! xh_isend(&it); xh_next(&it)) {
+    if (it.e->key == 0)
+      continue;
     free((void *)it.e->val);
   }
   free(pic->sym_names);