How to define a date in order to import an empty date from a CSV file?

I have a CSV file containing amongst other things a couple of date columns. Some of the date values are empty - that is just "_{" - a comma followed by another comma.}

The date rows in the table are defined like this: `PTY_USR_PASSWORD_DATE` DATE NULL DEFAULT NULL,

But when I import the data (using HeidiSQL), I get the following error: /* SQL Error (1292): Incorrect date value: '' for column 'PTY_USR_PASSWORD_DATE' at row 1 */

How should I set up the table to allow this? Or how can I modify the CSV?

John D.