diff --git a/lib/mini_mime.rb b/lib/mini_mime.rb
index 2d21da8..874a7d5 100644
--- a/lib/mini_mime.rb
+++ b/lib/mini_mime.rb
@@ -51,6 +51,7 @@ def binary?
 
   class Db
     LOCK = Mutex.new
+    @db = nil
 
     def self.db
       @db || LOCK.synchronize { @db ||= new }