diff --git a/lib/zip/output_stream.rb b/lib/zip/output_stream.rb
index 2f628931..266083cd 100644
--- a/lib/zip/output_stream.rb
+++ b/lib/zip/output_stream.rb
@@ -58,6 +58,7 @@ def open(file_name, encrypter = nil)
 
       # Same as #open but writes to a filestream instead
       def write_buffer(io = ::StringIO.new(''), encrypter = nil)
+        io.binmode if io.respond_to?(:binmode)
         zos = new(io, true, encrypter)
         yield zos
         zos.close_buffer
diff --git a/test/output_stream_test.rb b/test/output_stream_test.rb
index 40205076..61c9ac37 100644
--- a/test/output_stream_test.rb
+++ b/test/output_stream_test.rb
@@ -6,6 +6,8 @@ class ZipOutputStreamTest < MiniTest::Test
   TEST_ZIP = TestZipFile::TEST_ZIP2.clone
   TEST_ZIP.zip_name = 'test/data/generated/output.zip'
 
+  ASCII8BIT = 'ASCII-8BIT'
+
   def test_new
     zos = ::Zip::OutputStream.new(TEST_ZIP.zip_name)
     zos.comment = TEST_ZIP.comment
@@ -32,6 +34,15 @@ def test_write_buffer
     assert_test_zip_contents(TEST_ZIP)
   end
 
+  def test_write_buffer_binmode
+    io = ::StringIO.new('')
+    buffer = ::Zip::OutputStream.write_buffer(io) do |zos|
+      zos.comment = TEST_ZIP.comment
+      write_test_zip(zos)
+    end
+    assert_equal Encoding::ASCII_8BIT, buffer.external_encoding
+  end
+
   def test_write_buffer_with_temp_file
     tmp_file = Tempfile.new('')