using mutate in to update an existing value to null causes an IllegalArgumentException

//connect to Cluster

...

JObject o = JObject.FromObject(new

{

  title = "Sample",

  description = (string)null

});

await _Collection.InsertAsync<JObject>("test", o);

static void temp(MutateInSpecBuilder specs)

  {

    //any of the following lines will fail

    specs.Remove("title").Insert<string>("title", null);

    specs.Insert<string>("newKey", null, true);

    specs.Upsert<string>("title", null, true);

  }

await _Collection.MutateInAsync("test", temp);