mapping a date field when some records are blank

Summary

trying to map a posted date field from a bank file, but since some lines don't have posted date, it fails validation (issue date is brought in - that is the required date field)

Content

I have a bank file where I have an issue date (always populated) and a posted date (blank on some lines if check not yet cleared).  I need to bring in both columns, and don't want to set posted as text (lose the ability to filter).  

I've used this script - TO_DATE(UD5,'MM-DD-YY'), and it brings in the dates present, leaving blank the posted date when no value.  But, system doesn't validate, gives an error 'no source ud5'.