System.MissingMethodException

Hello:
Recently ,I create a WP8 DLL Plugins by VS2013.In this Plugins,I used a third-part dll.However,when i run my app with the plugins.it report an exception:System.MissingMethodException.I check my reference,it has no wrong.So,I don’t konw what happened.Can you help me?Thanks a lot!

When you create this plugin, do you use proper runtime? That is, is you dll build against .NET for Windows Phone 8 (and not 8.1).
The you should try to create a simple Windows Phone app and load it.