diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index af3578a0ee..be0570165e 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -508,6 +508,7 @@ if defined? Zlib
   class TestZlibGzipFile < Test::Unit::TestCase
     def test_gzip_reader_zcat
       Tempfile.create("test_zlib_gzip_file_to_io") {|t|
+        t.binmode
         gz = Zlib::GzipWriter.new(t)
         gz.print("foo")
         gz.close