diff --git a/lib/winrm/shells/base.rb b/lib/winrm/shells/base.rb
index 3d11ac49..87adac35 100644
--- a/lib/winrm/shells/base.rb
+++ b/lib/winrm/shells/base.rb
@@ -187,6 +187,7 @@ def remove_finalizer
       def self.finalize(connection_opts, transport, shell_id)
         proc { Thread.new { close_shell(connection_opts, transport, shell_id) } }
       end
+      private_class_method :finalize
     end
   end
 end