plpgsql related question: intervals and variables

Hello out there,

We have a problem in plpgsql:
We want to add variable periods (result of a query) to a timestamp.

Our Code looks like this:

Begin

heute := ''today'';
Select Into vk ourcolumn From table where other = foo;
If vk > 0 Then
vk_txt := ''Vorkuehlung notwendig'';
ez := heute + interval ''vk days'';
Else
vk_txt := ''Keine Vorkuehlung vorgeschrieben'';
End if;

We get the following:
ERROR: Bad interval external representation 'vk days'

The variable 'heute' is declared as timestamp,
'vk' as integer!

What have we done wrong??

:(

Thanks in advance,

Willi, Albin

--
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Wilhelm Graiss
Altirdning 12
8952 Irdning
03682/22451/267