How do I convert Union and Null as Col Sql query to Linq?

Question

SELECT *, NULL AS purchaseOrderItemId, NULL AS isOrdered, NULL AS orderedDate, NULL AS orderNote FROM t1 UNION SELECT *, NULL AS orderItemId FROM t2

I want to convert this to Linq query.

This is what I got so far, but getting null casting error.

 var query = DBContext.t1
                    .Select( x => x, new
                    {
                        purchaseOrderItemId = null,
                        isOrdered = null,
                        orderedDate = null,
                        orderNote = null
                    })
                    .Union(
                    DBContext.t2
                    .Select( x => x, new
                    {
                        orderItemId = null
                    }
                    )

How can I do?