Can't seem to get Generic Covariance/Contravariance to work

Suppose I have the following code:

public interface IBaseMessage { }

public interface IMessageProcessor<in T> where T : IBaseMessage {
            void Process(T msg);
        }

public class RRMessage : IBaseMessage {
          //something here
        }

public class BaseMessageProcessor {
         //something here
        }

public class RRMessageProcessor : BaseMessageProcessor, IMessageProcessor<RRMessage> {
            public void Process(RRMessage msg) {
                Console.WriteLine("Processed RRMessage");
            }
        }

public Dictionary<Type, IMessageProcessor<IBaseMessage>> MessageProcessors = new Dictionary<Type, IMessageProcessor<IBaseMessage>>();

[Test]
public void Test1() {
     var msgProcessor = new RRMessageProcessor();
     MessageProcessors.Add(typeof(RRMessage), msgProcessor);
     }

I enabled contravariance for the interface IMessageProcessor. Why does MessageProcessors.Add(typeof(RRMessage), msgProcessor); cause a compile time error:

Argument 2: cannot convert from 'RRMessageProcessor' to 'IMessageProcessor<IBaseMessage>'

It seems like it should be able to convert because RRMessageProcessor:IMessageProcessor<RRMessage:IBaseMessage>> How can I get this to work?