How can I use this subselect properly in an update? - Mailing list pgsql-sql

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)?