Q: using WHEN in SQL*Loader?

I need some help with how to use WHEN to load rows with an "empty" field in a comma-separated file?

Example extract from the data file

---
id1,surname1,,lastname1,mail1
id2,surname2,lastname2b,lastname2,mail2
--

I simply want to skip all rows with two lastnames. I have
tried using WHEN column3 = BLANKS (or '') but all rows
failed. And anything else I try also fails... so I'm
out of ideas.

 /Frederick