diff --git a/ChangeLog b/ChangeLog
index 7a9587df42..96d1f92299 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Jul 29 22:04:45 2013  Masaki Matsushita  <glass.saga@gmail.com>
+
+	* hash.c (rb_hash_compare_by_id): add function prototype.
+
 Mon Jul 29 21:53:41 2013  Masaki Matsushita  <glass.saga@gmail.com>
 
 	* hash.c (rb_hash_compare_by_id): don't call rb_hash_rehash()
diff --git a/hash.c b/hash.c
index 70b6bd4ac9..91a4e9fea0 100644
--- a/hash.c
+++ b/hash.c
@@ -2288,6 +2288,8 @@ rb_hash_flatten(int argc, VALUE *argv, VALUE hash)
     return ary;
 }
 
+static VALUE rb_hash_compare_by_id_p(VALUE hash);
+
 /*
  *  call-seq:
  *     hsh.compare_by_identity -> hsh