PUN + Android build error

Hi,

I'm using Unity 5.4.0f3 Personal. I just finished an iOS game and everything worked wonderfully. However, I changed to Android and when I build it returns me this error:

Connect() to 'ns.exitgames.com' () failed: System.TypeInitializationException: An exception was thrown by the type initializer for System.Net.Dns ---> System.TypeInitializationException: An exception was thrown by the type initializer for System.Net.Sockets.Socket ---> System.Configuration.ConfigurationErrorsException: Error Initializing the configuration system. ---> System.MissingMethodException: Method not found: 'Default constructor not found...ctor() of System.Configuration.ExeConfigurationHost'.

What am I missing?