I must be missing something.

It seems like it would be as easy as this.

bucket.PredicateExpression.AddWithAnd(PredicateFactory.CompareNotNull(BillableItemFieldIndex.InvoiceId));

But there is no CompareNotNull method.

Thanks Ren

Use the overload of CompareNull which accepts negate and set that parameter to true

Thanks! I knew it was something I was missing.