This code is giving the error "field 'fieldname' does not accept null values". Why not?

IF ! USED('inv_this_trans')
set null on
SELECT inventory.stocknum, ;
LEFT(SUBSTR(inventory.p_descript,1,AT("$",inventory.p_descript,2)+8),50) AS memotxt, ;
inventory.trans_id ;
FROM Tomparco!inventory AS inv_this_trans ;
INTO CURSOR tempcursor ;
ORDER BY stocknum

USE DBF("tempcursor") IN 0 AGAIN NODATA ALIAS inv_this_trans
USE IN tempcursor
ZAP IN inv_this_trans
ENDIF

Thanks
JD