Crystal Reports Checking for NULL date

Message

   I have a formula (crystal syntax) in a report. In this formula, I have to check the mont of a date field but this date sometime contains a NULL value.

If I remove the comment from the line containing "month({UnitValueCurrent.Date})", I receive an error saying that a date value must be provided.

Any ideas on how I can handle this?

My current formula:

if (not {?ShowSRUnitValues}) then //or IsNull({UnitValueCurrent.Date}) then
    0
else
    if {@ReturnCDN} > 0 then
        {UnitValueCurrent.ClassA} * ( 1 + ( {@ReturnCDN} / 100 * .8) )
    else
//        if not ( month({UnitValueCurrent.Date}) = 12 ) then
//            {UnitValueCurrent.ClassA}
//        else
            {UnitValueCurrent.ClassA} * ( 1 - ( {@ReturnCDN} / 100 * .8) )