diff --git a/hash.c b/hash.c
index 14f3b038f9..608738aab5 100644
--- a/hash.c
+++ b/hash.c
@@ -1723,14 +1723,14 @@ tbl_update(VALUE hash, VALUE key, tbl_update_func func, st_data_t optional_arg)
         .func = func,
         .hash = hash,
         .key  = key,
-        .value = (VALUE)optional_arg,
+        .value = 0
     };
 
     int ret = rb_hash_stlike_update(hash, key, tbl_update_modify, (st_data_t)&arg);
 
     /* write barrier */
     RB_OBJ_WRITTEN(hash, Qundef, arg.key);
-    RB_OBJ_WRITTEN(hash, Qundef, arg.value);
+    if (arg.value) RB_OBJ_WRITTEN(hash, Qundef, arg.value);
 
     return ret;
 }
diff --git a/test/ruby/test_hash.rb b/test/ruby/test_hash.rb
index e75cdfe7f9..7b8cf1c6c4 100644
--- a/test/ruby/test_hash.rb
+++ b/test/ruby/test_hash.rb
@@ -2355,6 +2355,11 @@ class TestHashOnly < Test::Unit::TestCase
     end
   end
 
+  def test_bug_21357
+    h = {x: []}.merge(x: nil) { |_k, v1, _v2| v1 }
+    assert_equal({x: []}, h)
+  end
+
   def test_any_hash_fixable
     20.times do
       assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")