Hello everyone.
Here is the select:

SELECT MIN(Who_Na) as Post_Na FROM Dok1 WHERE Td=38 ;
AND Dok1.D>o_Us.Otchet_d ORDER BY 1 INTO CURSOR Postavshik READWRITE 

When the select finds no matching records in the base table (Dok1) it still returns a result set with one record which contains the null. I see no reason why would one get something out of nothing.
Anyway, is there a universal way to not allow this happen? I had to make the cursor ReadWrite and then to
Replace all Post_Na with Nvl(Post_Na, '') && a kludge!

The number of my selects with aggregate functions is forbiddingly large.
Until your answer comes i will try not to think about the amount of code to dig thru.

All the best.
Kamil