How to avoid warning "null to Date"

Hello,

I have a field in DS which is a DateTime datatype. Sometimes, the date arrives as Null and DS tries to convert that 'null' value to the date format as <dd-Mon-yyyy>.

Anybody could explain the way to avoid the following warning message during the job execution? I thought to fix it with a isValidDate() function wrapping the output field but it's beaviour wasn't as I expected.

1240    10068    RUN-053006    12/12/2013 14:50:34    Cannot convert string <> to date using format string <dd-Mon-yyyy>.

1240    10068    RUN-053006    12/12/2013 14:50:34    |Data flow 17__RAW_LOAD|Transform User_detail_loader

Thank you very much in advance folks,

Regards,

nma