diff --git a/test/ruby/test_numeric.rb b/test/ruby/test_numeric.rb
index ecf5ebb32c..44c12ede9c 100644
--- a/test/ruby/test_numeric.rb
+++ b/test/ruby/test_numeric.rb
@@ -1,4 +1,5 @@
 require 'test/unit'
+require_relative 'envutil'
 
 class TestNumeric < Test::Unit::TestCase
   class DummyNumeric < Numeric
@@ -20,10 +21,18 @@ class TestNumeric < Test::Unit::TestCase
     assert_raise_with_message(TypeError, /can't be coerced into /) {1|:foo}
     assert_raise_with_message(TypeError, /can't be coerced into /) {1^:foo}
 
-    assert_raise_with_message(TypeError, /:\u{3042}/) {1+:"\u{3042}"}
-    assert_raise_with_message(TypeError, /:\u{3042}/) {1&:"\u{3042}"}
-    assert_raise_with_message(TypeError, /:\u{3042}/) {1|:"\u{3042}"}
-    assert_raise_with_message(TypeError, /:\u{3042}/) {1^:"\u{3042}"}
+    EnvUtil.with_default_external(Encoding::UTF_8) do
+      assert_raise_with_message(TypeError, /:\u{3042}/) {1+:"\u{3042}"}
+      assert_raise_with_message(TypeError, /:\u{3042}/) {1&:"\u{3042}"}
+      assert_raise_with_message(TypeError, /:\u{3042}/) {1|:"\u{3042}"}
+      assert_raise_with_message(TypeError, /:\u{3042}/) {1^:"\u{3042}"}
+    end
+    EnvUtil.with_default_external(Encoding::US_ASCII) do
+      assert_raise_with_message(TypeError, /:"\\u3042"/) {1+:"\u{3042}"}
+      assert_raise_with_message(TypeError, /:"\\u3042"/) {1&:"\u{3042}"}
+      assert_raise_with_message(TypeError, /:"\\u3042"/) {1|:"\u{3042}"}
+      assert_raise_with_message(TypeError, /:"\\u3042"/) {1^:"\u{3042}"}
+    end
   end
 
   def test_dummynumeric