How can I use this subselect properly in an update?

select
substring(t1.called_number from t2.offset),
t1.called_number
from
calls_join as t1,
john_prefix as t2
where
strpos(t1.called_number, t2.prefix) = '1'
order by length(t2.prefix) desc limit '1' ;

I'm having trouble using this as a sub-select in an UPDATE. Most of the
time it works. Then sometimes it doesn't.
Is there an issue with possible NULL values that can be found on table1
(t1)?