无法确定表达式错误的序列化信息

Question

尝试从数据库（Mongo DB）中删除值时出现Unable to determine the serialization information for the expression: (PlaceManager pm) => pm.FreePlaces.get_Item(index)错误。

这是我的代码：

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

这里是PlaceManager课程：

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

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

}