[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