Computed Field -> retrieve value of selection box with taxonomy terms

Hi,

I have a CCK field based on a taxonomy, say field_a.
I create a CCK Computed Field in which I want to retrieve the value that is selected in the previous field.

I tried:
$node_field[0]['value'] = $node->field_a[0]['tid'];

and

$node_field[0]['value'] = $node->field_a[0]['name'];

and

$node_field[0]['value'] = $node->field_a[0]['value'];

All without succes. When looking at the table content_type_mycontenttype I assume the first version with ['tid'] should work.

Is there a way to retrieve the information of a selection box in a computed field? Am I doing something wrong?

Regards,
Frank