1 files changed, 5 insertions, 3 deletions
diff --git a/src/hash.c b/src/hash.c
index 0bda2b48b..42b40482a 100644
--- a/src/hash.c
+++ b/src/hash.c
@@ -104,9 +104,11 @@ static void mrb_hash_modify(mrb_state *mrb, mrb_value hash);
 static inline mrb_value
 mrb_hash_ht_key(mrb_state *mrb, mrb_value key)
 {
-  if (mrb_string_p(key))
-    return mrb_str_dup(mrb, key);
-  else
+  if (mrb_string_p(key)) {
+    key = mrb_str_dup(mrb, key);
+    RSTR_SET_FROZEN_FLAG(mrb_str_ptr(key));
+    return key;
+  } else
     return key;
 }