more than one row returned by a subquery used as an expression

I'm trying to get this query to work:

update sectors set companies =(select companies from
industries where sector_id =sectors.id);

PG returns:

ERROR: more than one row returned by a subquery used
as an expression

Column companies is just a count of rows in the
related companies table. Queries like this worked:

update industries set companies =(select count(id)
from companies where industry_id =industries.id);

I know I could do a join in the subselect, but I'm
curious why this doesn't work. If I do the subselect
by itself, it looks like all of the rows from the
industries table are return (I expected an error). Is
this the "implicit FROM" gotcha?

CSN

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