[SQL] empty data

> serbr=> CREATE TABLE foo (mtdata date);
> CREATE
> serbr=> \dt
>
> Database = serbr
> +------------------+----------------------------------+----------+
> | Owner | Relation | Type |
> +------------------+----------------------------------+----------+
> | serbr | foo | table |
> +------------------+----------------------------------+----------+
> serbr=> INSERT INTO foo VALUES ('');
> ERROR: Bad date external representation
> serbr=> INSERT INTO foo VALUES ('//');
> ERROR: Bad date external representation

My problem arise in a more general context, where the date may not
be available at the time of data insert/update, and the data must
be empty.

What can I do? (I would not like to define a my own date-type...)

Sergio