diff --git a/ext/zlib/extconf.rb b/ext/zlib/extconf.rb
index 2b2dbb1..2187ed1 100644
--- a/ext/zlib/extconf.rb
+++ b/ext/zlib/extconf.rb
@@ -114,7 +114,7 @@
   Logging::message "#{OS_NAMES[os_code]}\n"
   defines << "OS_CODE=#{os_code}"
 
-  $defs.concat(defines.collect{|d|' -D'+d})
+  $defs.concat(defines.collect{|d|'-D'+d})
 
   if zsrc
     $defs << "-DHAVE_CRC32_COMBINE"
diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index 71159a0..38f39e5 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -677,6 +677,7 @@ def test_os_code
           assert_equal(Zlib::OS_CODE, f.os_code)
         end
       }
+      assert_equal(Zlib::OS_WIN32, Zlib::OS_CODE) if /mswin|mingw/ =~ RUBY_PLATFORM
     end
 
     def test_orig_name