Update From Temp Table

I'm trying to update a table from a temporary table but I'm struggling a little with the syntax. Here's what I have so far:

DECLARE @WeekNo INT
SET @WeekNo = 1

WHILE @WeekNo <= 53
BEGIN
SET @SQL = 'UPDATE [Targets]
SET [Target] = SELECT ISNULL(Week' + Cast(@WeekNo as VarChar(2)) + ', 0) FROM #targets
WHERE [CampaignID] = @CampaignID'
SET @WeekNo = @WeekNo + 1
EXECUTE (@SQL)
END

My temp table gets created OK, it's just setting the 'Target' field to the value of the same field in my temp table.

I get the following errors:
Msg 156, Level 15, State 1, Line 2
Incorrect syntax near the keyword 'SELECT'.
Msg 102, Level 15, State 1, Line 2
Incorrect syntax near ' + Cast(@WeekNo as VarChar(2)) + '.

Can anyone help?

Thanks in advance,
Brett