diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index 3f28d55ac1..6b0bc4de5e 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -1331,8 +1331,8 @@ class TestM17N < Test::Unit::TestCase
       env_encoding = Encoding.find("locale")
     end
     ENV.each {|k, v|
-      assert_equal(env_encoding, k.encoding, k)
-      assert_equal(env_encoding, v.encoding, v)
+      assert_equal(env_encoding, k.encoding, proc {"key(#{k.encoding})=#{k.dump}"})
+      assert_equal(env_encoding, v.encoding, proc {"key(#{k.encoding})=#{k.dump}\n" "value(#{v.encoding})=#{v.dump}"})
     }
   end