diff --git a/test/plugin/test_out_file.rb b/test/plugin/test_out_file.rb
index 0e9dd17f98..062ba374d7 100644
--- a/test/plugin/test_out_file.rb
+++ b/test/plugin/test_out_file.rb
@@ -334,7 +334,7 @@ def create_driver(conf = CONFIG, opts = {})
       assert_equal r5, d.formatted[4]
 
       read_gunzip = ->(path){
-        File.open(path){ |fio|
+        File.open(path, 'rb'){ |fio|
           Zlib::GzipReader.new(StringIO.new(fio.read)).read
         }
       }