diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 8b7c3363d1..71299181ec 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -677,7 +677,7 @@ EOT
   end
 
   def test_getc_invalid3
-    with_pipe("utf-16le:euc-jp") {|r, w|
+    with_pipe("utf-16le:euc-jp", binmode: true) {|r, w|
       before1 = "\x42\x30".force_encoding("utf-16le")
       before2 = "\x44\x30".force_encoding("utf-16le")
       invalid = "\x00\xd8".force_encoding("utf-16le")