LiteDB: Error "Invalid format: Collection" when doing Upsert

I am trying to do an “Collection.Upsert” on the following POCO class. [There are other classes like this where upsert works fine!. Only this class fails]. The error is: Error “Invalid format: Collection” when doing Upsert

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public class Order : BindableBase { public Order() { this.OrderItems = new List<string>(); } public int Id { get; set; } [BsonRef(“OrderMaster”)] public OrderMaster OrderMaster { get; set; } public List<string> OrderItems { get; set; } }

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Stack trace:

Invalid format: Collection at LiteDB.CollectionService.Add (System.String name) [0x0002b] in <b99fd1821def4ff9a5794b6636ed75ad>:0 at LiteDB.LiteEngine.GetCollectionPage (System.String name, System.Boolean addIfNotExits) [0x00035] in <b99fd1821def4ff9a5794b6636ed75ad>:0 at LiteDB.LiteEngine.Transaction[T] (System.String collection, System.Boolean addIfNotExists, System.Func`2[T,TResult] action) [0x00006] in <b99fd1821def4ff9a5794b6636ed75ad>:0

Thank you for the help…