HELP with COMPLEX COUNT/CASE

Hey guys, I am having trouble with the following:
COUNT( CASE WHEN xferleads.timestamp >= wo_date
AND xferleads.timestamp <= wo_end_date
THEN xferleads.customer
ELSE NULL END ) AS countslw

This works well except under one condition:
There are times when wo_end_date is NULL and under such a condition the count is obviously 0

I assume the solution is a nested CASE but I am having trouble with the syntax.

In other words - count how many xferleads.timestamp there are between wo_date and wo_end_date. If wo_end_date is NULL, count SINCE wo_date only.

when wo_end_date is null, substitute xferleads.timestamp, which is always going to be less than or equal to itself

thank you.