fejl i storedprocedure (sub query), som alligevel virker fint :S

hejsa,
jeg får følgende fejl når jeg kører min stored procedure:

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

Selve koden bag proceduren ser sådan ud:

ALTER PROCEDURE dbo.SinglePurchaseDealStatus_Update

    (
        @SerialNumber nvarchar(18),
        @Status int,
        @ApproveDate1 datetime,
        @ApproveDate2 datetime,
        @ApproveDate3 datetime,
        @ClosedDate datetime,
        @WorkerID int,
        @ManagerID int,
        @ProviderID int,
        @ClosedID int,
        @RejectionText ntext,
        @CancelStatus int,
        @TerminationStatus int
    )

AS

BEGIN TRAN

    UPDATE
        SinglePurchaseDealStatus
    SET
        Status        = @Status,
        ApproveDate1  = @ApproveDate1,
        ApproveDate2  = @ApproveDate2,
        ApproveDate3  = @ApproveDate3,
        ClosedDate    = @ClosedDate,
        WorkerID      = @WorkerID,
        ManagerID    = @ManagerID,
        ProviderID    = @ProviderID,
        ClosedID      = @ClosedID,
        RejectionText = @RejectionText,
        CancelStatus  = @CancelStatus,
        TerminationStatus = @TerminationStatus
    WHERE
        SerialNumber = @SerialNumber


    IF @Status = 3
    BEGIN
        UPDATE
            KOMMAProject
        SET       
            ReservedSeats = ReservedSeats+1   
        WHERE
            KOMMAProjectID IN (SELECT KOMMAProjectID FROM ServicePurchase WHERE SerialNumber = @SerialNumber)
    END
   
    DECLARE @TerminationCauseID int
    DECLARE @EndingCauseID int
   
    SET @TerminationCauseID = (SELECT TerminationCauseID FROM ServicePurchase WHERE SerialNumber = @SerialNumber)
    SET @EndingCauseID = (SELECT EndingCauseID FROM ServicePurchase WHERE SerialNumber = @SerialNumber)
   
    IF  @Status > 3 AND @TerminationCauseID = -1 AND @EndingCauseID = -1
    BEGIN
        UPDATE
            KOMMAProject
        SET       
            ReservedSeats = ReservedSeats - 1
        WHERE
            KOMMAProjectID IN (SELECT KOMMAProjectID FROM ServicePurchase WHERE SerialNumber = @SerialNumber)
    END


IF @@ERROR <> 0
    ROLLBACK TRAN
ELSE
    COMMIT TRAN
GO

Hvad er det der driller? Som sagt så bliver status opdateret og det hele kører fint nok, undtaget den nævnte fejl meddelse.

På forhånd tak