diff --git a/test/test_detect_general.rb b/test/test_detect_general.rb
index c7f7d6f..d7a65ef 100644
--- a/test/test_detect_general.rb
+++ b/test/test_detect_general.rb
@@ -27,6 +27,9 @@ class TestDetectGeneral < Test::Unit::TestCase
   def setup
     Locale.init
     Locale.clear_all
+    if Locale::Driver.const_defined?(:Win32)
+      Locale::Driver::Win32.set_thread_locale_id(0xffff) # invalid
+    end
     ENV["LC_ALL"] = nil
     ENV["LC_CTYPE"] = nil
     ENV["LC_MESSAGES"] = nil
@@ -34,6 +37,12 @@ def setup
     ENV["LANGUAGE"] = nil
   end
 
+  def teardown
+    if Locale::Driver.const_defined?(:Win32)
+      Locale::Driver::Win32.set_thread_locale_id(nil)
+    end
+  end
+
   def test_lc_all
     ENV["LC_ALL"] = "ja_JP.eucJP"
     ENV["LC_CTYPE"] = "fr_FR.ISO-8859-1"  #Ignored.