NullReferenceException from wcf service

I have a wcf service in which I use the Miquels Castro method, and not a user, the link to the service creates VS.NET 2010.

Everything worked fine until I added a new method. When I added a new method, I received an error message:

The first random exception of type 'System.ServiceModel.FaultException`1' occurred in mscorlib.dll

Additional information: The object link is not installed in the instance object.

Then

An unhandled exception of type 'System.Runtime.Serialization.SerializationException' occurred in the Unknown module.

Additional information: Type 'System.ServiceModel.ExceptionDetail' in Assembly 'System.ServiceModel, Version = 4.0.0.0, Culture = Neutral, PublicKeyToken = b77a5c561934e089' is not marked as serializable.

-:

public DataTable GetResearchSellingTitles(string network, string year, 
    string quarter)
{
    return Channel.GetResearchSellingTitles(network, year, quarter);
}

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