How to combine 2different IQueryable/List/Collection with same base class? LINQ Union and Covariance issues

I am trying to combine (union or concat) two lists/collection into one. The two lists have a common base class. e.g. I've tried this:

        IQueryable<ContractItem> contractItems = myRepository.RetrieveContractItems();
        IQueryable<ChangeOrderItem> changeOrderItems = myRepository.RetrieveChangeOrderItems();

        IQueryable<ItemBase> folderItems = contractItems.Concat<ItemBase>(changeOrderItems);

But am getting the LINQ error DbUnionAllExpression requires arguments with compatible collection ResultTypes.

Anybody know how to do this properly? The only thing I could google was another StackOverflow question: LINQ Union objects with same Base Class

Thanks.