TPT: using VARDATE and empty string

Hello,
when I used VARDATE  in TPT transformation everythink worked perfectly until the DATE column was filled in the source extract.
First row containing empty date field causes error:
DATACON_O2EV: TPT10672 Error: Data contains a separator character which is unmatched by the formatting string.

DATACON_O2EV: TPT19003 TPT Exit code set to 12.

Is it correct?

How can I convert DATE with empty / undefined fields in TPT using VARDATE functionality?

Thanks

         Tom