NServiceBus won't serialize this properly

gistfile1.cs

    public class MyMessage : IMessage
    {
        public string OneValue { get; set; }

        public PropertyBase[] Properties { get; set; }
    }

    public class PropertyBase
    {
    }

    public class FirstProperty : PropertyBase
    {
        public string AnotherValue { get; set; }
    }

    public class SecondProperty : PropertyBase
    {
        public string YetAnotherValue { get; set; }
    }