diff --git a/lib/smart_proxy_dynflow/io_buffer.rb b/lib/smart_proxy_dynflow/io_buffer.rb
index e22e7ea..e0ffe91 100644
--- a/lib/smart_proxy_dynflow/io_buffer.rb
+++ b/lib/smart_proxy_dynflow/io_buffer.rb
@@ -88,7 +88,7 @@ def read_available!
       def write_available!
         until @buffer.empty?
           n = @io.write_nonblock(@buffer)
-          @buffer = @buffer[n..-1]
+          @buffer = @buffer.bytes.drop(n).pack('c*')
         end
       rescue IO::WaitWritable # rubocop:disable Lint/SuppressedException
       rescue EOFError
diff --git a/test/io_buffer_test.rb b/test/io_buffer_test.rb
index bc01a96..d31f01d 100644
--- a/test/io_buffer_test.rb
+++ b/test/io_buffer_test.rb
@@ -60,6 +60,12 @@ class CustomWaitWritable < RuntimeError
           buffer.write_available!
           assert_equal buffer.to_s, 'ello'
         end
+
+        it 'can deal with wide characters' do
+          buffer.add_data('ラジオで勉強しました')
+          buffer.write_available! # This would raise an exception
+          assert_equal buffer.to_s, ''
+        end
       end
 
       describe '#read_available!' do