Primary Key On Combination Of Nullable Fields, At Least One Not-null

I have a case where a table has two candidate primary keys,
but either (but not both) may be NULL. I don't want to store
a copy of the concatenated ISNULL'ed fields as an additional
column, though that would work if necessary. Instead, I tried
the following (this is a related simplified example, not my
real one):

CREATE FUNCTION ApplyActionPK(
@IP int = NULL,
@DNS varchar(64) = NULL
)
RETURNS varchar(74) -- NOT NULL
AS
BEGIN
declare @val varchar(74)
set @val = str(ISNULL(@IP, 0), 10)
set @val = @val + ISNULL(@DNS, '')
return @val
-- Also tried "return str(ISNULL(@IP, 0), 10)+ISNULL(@DNS, '')"
-- Also tried "return ISNULL(STR(@IP, 10), ISNULL(@DNS, ''))"
-- ... and other things...
END
GO

create table ApplyAction(-- An action applies to a computer
Act varchar(16) NOT NULL,-- The action to apply
IP int NULL,-- The computer IP address, or
DNS varchar(64) NULL,-- The DNS name of the computer
Target as dbo.ApplyActionPK(ComputerID, DNS), -- PK value
-- Also tried "Target as ISNULL(STR(@IP, 10), ISNULL(@DNS, ''))"
CONSTRAINT PK_ApplyAction PRIMARY KEY(Act, Target)
)

SQL Server always complains that the primary key constraint cannot be
created over a nullable field - even though in no case will the 'Target'
field be NULL.

Please don't explain that I should store an IP address as a string.
Though that would suffice for this example, it doesn't solve my
actual problem (where there are four nullable fields, two of which
are FKs into other tables).

What's the reason for SQL Server deciding that the value is NULLable?
What's the usual way of handling such alternate PKs?

Clifford Heath.