Subquery Assistance Needed

I am getting the following syntax error:

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

From my query: (Please note the Where clause values will look odd to you, however they are not broken or wrong (its how the system reads it (odd I know)). The only issue I have is the subquery.

/*Schedule Summary*/
SET NOCOUNT ON

--Patient Appointments

DECLARE @Today DATETIME
DECLARE @Tomorrow DATETIME
SET @Today = '10/29/2007'
SET @Tomorrow = dateadd(d, 1, '10/29/2007')

SELECT Date=convert(datetime,convert(char(12),Start,1)),
convert(datetime,aps.Start) AS ResourceStart,
convert(datetime,aps.Stop) AS ApptStop,
ApptTime = datediff(minute, aps.Start, aps.Stop),
df.Listname AS Resource,
Facility= f.ListName,
d.Listname AS DoctorName,
--'Available / No Appt' AS Type,
(SELECT dbo.sfnGetAllocsForSlot(aps.ApptSlotId)
FROM ApptSlot aps
JOIN Schedule s ON aps.ScheduleId = s.ScheduleId)AS TYPE,
'Available' AS 'Patient Name',
1 as ApptKind

FROM ApptSlot aps
JOIN Schedule s ON aps.ScheduleId = s.ScheduleId
JOIN DoctorFacility df ON s.DoctorResourceId = df.DoctorFacilityId
JOIN DoctorFacility f ON aps.FacilityId = f.DoctorFacilityId
JOIN DoctorFacility d ON s.DoctorResourceId = d.DoctorFacilityId
LEFT JOIN Appointments a ON aps.apptid = a.appointmentsID

WHERE --Filter on resource
(
('7' IS NOT NULL AND df.DoctorFacilityId IN (7)) OR
('7' IS NULL)
)
AND
(
(NULL IS NOT NULL AND aps.FacilityId IN (NULL)) OR
(NULL IS NULL)
)
AND (Start >= @Today OR @Today IS NULL)
AND (Start < @Tomorrow OR @Tomorrow IS NULL)
AND ApptId IS NULL
AND APS.ListOrder <> -1

ORDER BY [Resource], [ResourceStart]