Problems with Windows Phone 8.1 plugins

I am having problems when I try to develop plugins for WP 8.1 I build my fake dll and WP 8.1 class library. When I put my real plugin to

Assets\Plugins\WP81 I am getting this error:

*Error 1 Internal compiler error. See the console log for more information. output was:
Unhandled Exception: System.IO.FileNotFoundException: Could not load file or assembly ‘System.Runtime, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’ or one of its dependencies.
File name: ‘System.Runtime, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’
at (wrapper managed-to-native) System.Reflection.Assembly:InternalGetType (

It works for WP 8.0 though. Is WP 8.1 plugins not supported, has anyone tried ?

I found the problem. I should put it under Plugins/Metro not Plugins/WP81