My guess is that something went mildly south for an entirely different reason. Some SQL variants return NULL when there is a decimal over-run (a nasty habit).
I'd suggest using the more standard form of:
, Sum(b.revenue) -- Will never be NULL unless ALL values are NULL
FROM tableA AS a
JOIN tableB AS b -- This might need to be a LEFT JOIN
ON (b.account = a.account) -- Is this assumption valid?
GROUP BY a.account -- This seems to be required
ORDER BY a.account -- This is just for convenience
Give that a shot and see if it is closer to what you want!