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.