SQL - Order by Asc query

SQL 2005 / 2008

This query over here ...

select DateFilled from
                  (
                  select patcustid,DaysSupply,DateFilled from vOeOrderHistoryWide
                  union all
                  select patcustid,DaysSupply,DateFilled from vOeOrderWide
                  )as x
                  where patcustid = 8

Returns Null and Date Field.

When i filter the query with Order By Asc - It returns NULL. Instead of picking the NULL Value, i am looking for first lowest Date Filled.

How to tune the query to fetch the lowest Date Filled !!

select top 1 DateFilled from
			(
			select patcustid,DaysSupply,DateFilled from vOeOrderHistoryWide
			union all
			select patcustid,DaysSupply,DateFilled from vOeOrderWide
			)as x
			where patcustid = 8
			order by DateFilled asc

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