Windows Phone: WWW(String,Byte[],Hashtable) doesn't exist in target framework.

Hello, I am trying to build my project on Windows Phone. Everything works fine on Android and iOS but on Windows Phone, I get this error: > Error building Player: Exception: Error: method `System.Void UnityEngine.WWW::.ctor(System.String,System.Byte[],System.Collections.Hashtable)` doesn't exist in target framework. Does someone as any clue? Problem seems also occurs when exporting to Windows 8 application (not desktop).