Why am I unable to mock a private field of singleton object?

The following test code results in this error message: "Instance passed is a handle to a future fake, but no instance was created"

How do I fix this?

My real aim is to replace all future instances of the Singleton class with my own instance. Am I going about this the wrong way?

Class I am trying to fake:

public sealed class Singleton

{

        ....

        private Dictionary<string, Agent> _dictAgent = new Dictionary<string, Agent>();

}

My test code (abbreviated):

        [TestMethod, Isolated]

        public void CheckpointTest4()

        {