SELECT DISTINCT And ORDER BY With Aliased Column

This query demonstrates a problem I have run across:


USE AdventureWorks

GO

-- This query works fine.

SELECT DISTINCT FirstName AS Name1 FROM Person.Contact ORDER BY FirstName

GO

-- This query also works fine.

SELECT ISNULL(FirstName, '') AS Name1 FROM Person.Contact ORDER BY FirstName

GO

-- This query returns error 145

SELECT DISTINCT ISNULL(FirstName, '') AS Name1 FROM Person.Contact ORDER BY FirstName

GO


The last query returns the error "ORDER BY items must appear in the select list if SELECT DISTINCT is specified".
It will work if I change ORDER BY to use "Name1" instead of "FirstName", but in the situation I have at hand, the query is generated by third-party software and I don't have the ability to change it.
Can anyone explain why what's going on here? Oddly, this same query will work if I run it against SQL Server 2000.