Problem med en collum i select

Hey,

Jeg får en fejl ved kørsel af følgende sproc:

set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
go

ALTER PROCEDURE [dbo].[usp_HitLog_GetSessionIDs]
    @Since DATETIME,
    @Until DATETIME
AS
BEGIN
    SET NOCOUNT ON;

    SELECT SessionID, Logged
    FROM tblHitLog
    WHERE Logged > @Since
    AND Logged < @Until
    GROUP BY SessionID
    ORDER BY Logged DESC
END


Fejl:

Msg 8120, Level 16, State 1, Procedure usp_HitLog_GetSessionIDs, Line 9
Column 'tblHitLog.Logged' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Hvad er der galt?