diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index 7713f07df2..3dccc76c91 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -1102,7 +1102,7 @@ class TestM17N < Test::Unit::TestCase
   end
 
   def test_str_concat
-    assert_equal(1, "".concat(0xA2))
+    assert_equal(1, "".concat(0xA2).size)
     assert_equal("A\x84\x31\xA4\x39".force_encoding("GB18030"),
                  "A".force_encoding("GB18030") << 0x8431A439)
   end