Error when deserializing Ion blob to nullable `Guid` type

[guyilin-amazon]

When the property has a nullable Guid type, i.e. Guid?, the object mapper will throw exception when trying to deserialize the Ion blob to Guid type.
Example class:

public class AnnotateGuidsClass 
{
    public Guid? NullableGuid { get; set; }
}

Exception:

System.ArgumentException: Object of type 'System.Byte[]' cannot be converted to type 'System.Nullable`1[System.Guid]'.