Warning: Null value is eliminated by an aggregate or other SET operation.???

Hi all

I have this query below when i run it gives a message:
Warning: Null value is eliminated by an aggregate or other SET operation.

How do I solve that warning?

Please kindly assist, thanks

SELECT
R1.JobNumber,
R1.JobName,
R1.ProjectManagerNumber,
CASE WHEN R1.LastTimeReg = '1900.01.01' THEN 'N/A' ELSE R1.LastTimeReg END AS LastTimeReg,
CASE WHEN R1.LastExpenseReg = '1900.01.01' THEN 'N/A' ELSE R1.LastExpenseReg END AS LastExpenseReg,
CASE WHEN R1.LastPOReg = '1900.01.01' THEN 'N/A' ELSE R1.LastPOReg END  AS LastPOReg

FROM
(SELECT
JH.JobNumber,
JH.JobName,
JH.ProjectManagerNumber,
ISNULL(MAX(TSL.PeriodStart), '1900.01.01') AS LastTimeReg,
ISNULL(MAX(ESL.DateOfEntry), '1900.01.01') AS LastExpenseReg,
ISNULL(MAX(RH.CreatedDate), '1900.01.01') AS LastPOReg

FROM
JobHeader JH LEFT OUTER JOIN TimeSheetLine TSL ON JH.JobNumber = TSL.JobNumber
LEFT OUTER JOIN ExpenseSheetLine ESL ON JH.JobNumber = ESL.JobNumber
LEFT OUTER JOIN RequisitionHeader RH ON JH.JobNumber = RH.JobNumber
WHERE
(JH.IsBlocked = 0 AND JH.BlockedForInvoicing = 0 AND JH.Closed = 0 AND JH.Template = 0)
GROUP BY
JH.JobNumber,
JH.JobName,
JH.ProjectManagerNumber
) AS R1

WHERE
R1.ProjectManagerNumber  = '300020'
AND (R1.LastTimeReg != '1900.01.01' OR R1.LastExpenseReg != '1900.01.01' OR R1.LastPOReg != '1900.01.01')
AND ((DateDiff(Day, R1.LastTimeReg, GetDate()) >= 28) AND (DateDiff(Day,R1.LastExpenseReg, GetDate()) >= 28) AND (DateDiff(Day, R1.LastPOReg, GetDate()) >= 28))