How to extend SoapException

Question

I am stuck trying to extend SoapException in order to add two additional string attributes.
I have a Web-Service method that should throw CustomSoapException derived from SoapException and I want to catch that CustomSoapException in web service client. But when I try to expose CustomSoapException to web service client by adding [XmlInclude(typeof(CustomSoapException))] property to my WebMethod, my ASMX web service fails upon start-up with the message:

Cannot serialize member System.Exception.Data of type System.Collections.IDictionary, because it implements IDictionary.

If someone can show me how to properly serialize Data property of IDictionary type inside of my CustomSoapException so it can be correctly exposed to web service client. I don't even intend to put any data inside Data property. Maybe it can be somehow removed completely from extended class altogether to avoid the need to serialize it.

Here's my code for CustomSoapException:

[Serializable]
public class CustomSoapException : SoapException, ISerializable
{
    private string customExceptionType;
    private string developerMessage;
    private string userMessage;

    public override System.Collections.IDictionary Data
    {
        get
        {
            return base.Data;
        }
    }

    public CustomSoapException(): base()
    {
    }

    public string GetDeveloperMessage()
    {
        return developerMessage;
    }
    public string GetCustomExType()
    {
        return customExceptionType;
    }

    void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)
    {
        base.GetObjectData(info, context);
    }

    public CustomSoapException(SerializationInfo info, StreamingContext context)
        : base(info, context)
    {
    }

    public CustomSoapException(string usrMessage, string devMessage, string customExType) : base(usrMessage, SoapException.ServerFaultCode)
    {
        customExceptionType = customExType;
        developerMessage = devMessage;
        userMessage = usrMessage;
    }
}

Here's my WebMethod code inside of asmx.cs file:

[WebMethod(EnableSession = true)]
[XmlInclude(typeof(CustomSoapException))]
public void testCustomExceptionPassing()
{
 throw new CustomSoapException("user message", "developer message","customException");
 }

Web service client code:

try
{
   Srv.testCustomExceptionPassing();
}
catch (SoapException ex) {
   string devMessage = (ex as Srv.CustomSoapException).GetDeveloperMessage();
}