Update Statement not working in SQL

Hi,

I need to update a column from the table when the datekey and tranchannel key matches, and as i am using sum i cannot directly do it i am using a subquery like this

UPDATE   EDW.Planning.TEDW_F_TRANSACTION_CHANNEL_OPS_GOAL 
SETOPS_GOAL = 
(
SelectSUM  ( TEDW_F_STORE_OPS_GOAL.OPS_GOAL) 
From EDW.Planning.TEDW_F_STORE_OPS_GOAL
INNER JOIN
              EDW.Calendar.TEDW_D_DATE_TYPE_1
ON            TEDW_D_DATE_TYPE_1.DATE_KEY                     = TEDW_F_STORE_OPS_GOAL.DATE_KEY
cross join
              EDW.Utility.TEDW_J_PROCESS_DATE 
WHERE TEDW_F_TRANSACTION_CHANNEL_OPS_GOAL.DATE_KEY=TEDW_D_DATE_TYPE_1.DATE_KEY
ANDTEDW_F_TRANSACTION_CHANNEL_OPS_GOAL.TRANSACTION_CHANNEL_KEY=   TEDW_F_STORE_OPS_GOAL.TRANSACTION_CHANNEL_KEY
AND     TEDW_F_STORE_OPS_GOAL.OPS_GOAL IS NOT NULL
)

But if i run this one i get an error saying 

Cannot insert the value NULL into column 'OPS_GOAL', table 'EDW.Planning.TEDW_F_TRANSACTION_CHANNEL_OPS_GOAL'; column does not allow nulls. UPDATE fails.

I got this errror so i included NOTNULL in the where clause but still i get the same.

Any Suggestions please?

Thanks.