1 files changed, 1 insertions, 0 deletions
diff --git a/src/state.c b/src/state.c
index 463fa9059..2f66e494b 100644
--- a/src/state.c
+++ b/src/state.c
@@ -237,6 +237,7 @@ mrb_str_pool(mrb_state *mrb, mrb_value str)
       ns->as.heap.ptr[len] = '\0';
     }
   }
+  MRB_SET_FROZEN_FLAG(ns);
   return mrb_obj_value(ns);
 }