MongoDB Unknown discriminator value => deserialize to JSonDocument

In MongoDB I've a class with a property MyProperty of type object.

public MyClass
{
    public object MyProperty;
    public string Prop1;
    public DateTime Prop2;
    public int Prop3;
}

Serialization to MongoDB works without any problem creating a JSon of this type:

MyProperty" : {
                "_t" : "ExampleClass",
            [...]
}

But when I try do seserialize it I get the following error:

An error occurred while deserializing the MyProperty property of class MyClass: Unknown discriminator value 'ExampleClass'.

I'd like to deserialize MyProperty to a simple generic BsonDocument or to a string.