diff --git a/lib/fluent/plugin/output.rb b/lib/fluent/plugin/output.rb
index c600e35287..d2107330ba 100644
--- a/lib/fluent/plugin/output.rb
+++ b/lib/fluent/plugin/output.rb
@@ -1036,17 +1036,17 @@ def generate_format_proc
       #   iteration of event stream, and it should be done just once even if total event stream size
       #   is bigger than chunk_limit_size because of performance.
       def handle_stream_with_custom_format(tag, es, enqueue: false)
-        meta_and_data = {}
+        meta_and_data = Hash.new { |h, k| h[k] = [] }
         records = 0
         es.each(unpacker: Fluent::MessagePackFactory.thread_local_msgpack_unpacker) do |time, record|
           meta = metadata(tag, time, record)
-          meta_and_data[meta] ||= []
           res = format(tag, time, record)
           if res
             meta_and_data[meta] << res
             records += 1
           end
         end
+        meta_and_data.default_proc = nil
         write_guard do
           @buffer.write(meta_and_data, enqueue: enqueue)
         end
@@ -1057,14 +1057,14 @@ def handle_stream_with_custom_format(tag, es, enqueue: false)
 
       def handle_stream_with_standard_format(tag, es, enqueue: false)
         format_proc = generate_format_proc
-        meta_and_data = {}
+        meta_and_data = Hash.new { |h, k| h[k] = MultiEventStream.new }
         records = 0
         es.each(unpacker: Fluent::MessagePackFactory.thread_local_msgpack_unpacker) do |time, record|
           meta = metadata(tag, time, record)
-          meta_and_data[meta] ||= MultiEventStream.new
           meta_and_data[meta].add(time, record)
           records += 1
         end
+        meta_and_data.default_proc = nil
         write_guard do
           @buffer.write(meta_and_data, format: format_proc, enqueue: enqueue)
         end