Unable to determine the serialization information for the expression error

I'm getting an Unable to determine the serialization information for the expression: (PlaceManager pm) => pm.FreePlaces.get_Item(index) error when trying to remove value from database (Mongo DB).

Here's my code:

var result = _placeManager.Update(Query.EQ("_id", 0),
                             Update<PlaceManager>.Pull(pm => pm.FreePlaces[index], placeId));

And here's PlaceManager class:

public class PlaceManager
{
    [BsonId]
    public int Id { get; set; }

    public int CurrentSize { get; set; }
    public Dictionary<int, List<int>> FreePlaces { get; set; }

}

Copyright Notice：Content Author:「Yury Pogrebnyak」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/13752901/unable-to-determine-the-serialization-information-for-the-expression-error