diff --git a/ChangeLog b/ChangeLog
index 8b84a003ec..8841434001 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Nov 13 15:40:43 2011  Tanaka Akira  <akr@fsij.org>
+
+	* ext/dbm/extconf.rb: duplicate $libs and $defs when save them.
+
 Sun Nov 13 12:43:48 2011  Tanaka Akira  <akr@fsij.org>
 
 	* ext/dbm/extconf.rb: rollback $libs and $defs when db detection is
diff --git a/ext/dbm/extconf.rb b/ext/dbm/extconf.rb
index f8139495d3..302b5517a5 100644
--- a/ext/dbm/extconf.rb
+++ b/ext/dbm/extconf.rb
@@ -29,8 +29,8 @@ headers.found = []
 headers.defs = nil
 
 def headers.db_check(db)
-  old_libs = $libs
-  old_defs = $defs
+  old_libs = $libs.dup
+  old_defs = $defs.dup
   result = db_check2(db)
   if !result
     $libs = old_libs