diff --git a/hash.c b/hash.c
index b68965e074..2142132597 100644
--- a/hash.c
+++ b/hash.c
@@ -1710,14 +1710,14 @@ 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 1fc5295707..6fa8087100 100644
--- a/test/ruby/test_hash.rb
+++ b/test/ruby/test_hash.rb
@@ -2315,6 +2315,11 @@ 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;'}")
diff --git a/version.h b/version.h
index 05452cb67a..3b8d50b4bf 100644
--- a/version.h
+++ b/version.h
@@ -11,7 +11,7 @@
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 8
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 154
+#define RUBY_PATCHLEVEL 155
 
 #include "ruby/version.h"
 #include "ruby/internal/abi.h"