ORA-01722: invalid number resulting from sql statement

 Hello, we are receiving an ORA-01722 Invalid Number message from the following select statement:

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select sr.id, sr.formid, sr.userid, sr.sid, sr.status, sr.startdate, sr.subm
itdate
,so.outcome_code, so.outcome_description, so.outcome_user, so.outcome_date
,so.status process_status
from our_user.ourform_slr_results sr, our_user.ourprocess_outcomes so
where sr.sid = '995387210'
and sr.formid = 1
and sr.status in ('Completed','In-Progress')
and so.process_code (+) = 'SLR'
and so.process_source (+) = 'ourform_slr_results'
and so.process_id (+) = sr.id
and (so.outcome_date = (select max(outcome_date) from our_user.ourprocess_ou