Filter out NULL values not working in OBIEE column formula

I have a column formula that I've almost, but not quite, gotten to work. The problem is that "Program Start Date" field is a VARCHAR in the format 201209 and is sometimes NULL, so the formula doesn't work when this field is NULL. I am able to filter out NULL values by creating an analysis, and when I do this the formula does work. But when I try to do the same thing by adding a condition to the column formula, it doesn't work.

SUM(CASE WHEN

"Enrolment - College"."F College Enrolment"."Program Start Date" is not NULL and

(TimeStampDiff(SQL_TSI_month, TO_DATETIME("Enrolment - College"."F College Enrolment"."Program Start Date" || '01', 'yyyy/mm/dd') , TO_DATETIME(LEFT("Enrolment - College"."D Time"."Fiscal Year", 4)||'/09/01', 'yyyy/mm/dd'))) / 12 >= 5

THEN 1 ELSE 0 END)