hey Brett,
you can try something like
if isnull({POTENTIAL_CLAIMANT_VIEW_MS.PATIENT_ID_NUM})
then '000000'
else
if {POTENTIAL_CLAIMANT_VIEW_MS.PATIENT_ID_NUM} = ''
then '000000'
else
{POTENTIAL_CLAIMANT_VIEW_MS.PATIENT_ID_NUM}
it could be that the field is just blank or nulls are being converted, and not NULL.
cheers,
jamie