mongodb use findall() the result cannot convert to list

public class RepairProject
{
        [MongoId]
        public String Id { get; set; }
        public String Name { get; set; }
        public List<List<String>> RepairSteps { get; set; }
}

var list = collection.FindAll();
List<RepairProject> lists = new List<RepairProject>();
lists.AddRange(list.Documents);
return lists;