MasterVB.NET

If I change MyAjaxClass sample (using SqlString istead of string) like that
...
private SqlString m_FirstName = "Michael";

public SqlString FirstName
{
get { return m_FirstName; }
set { m_FirstName = value; }
}
...

here is the method that pass to the client an object of type MyAjaxClass

[AjaxPro.AjaxMethod]
public MyAjaxClass GetClass()
{
MyAjaxClass c = new MyAjaxClass();
return c;
}

but when javascript method execute function GetClass_callback(response){...}
the response.value is null

What can I do to use classes with properties types like SqlString,
SqlInt32 etc. ?