Informix Sub Query: Case Statement

(
SELECT
    CASE
    WHEN IS NULL (sum (po_order_line.order_line_amnt))
        THEN 'ZERO'
    WHEN sum (po_order_line.order_line_amnt) > 0 AND sum (po_order_line.order_line_amnt) < 1000
        THEN 'LOW'
    WHEN sum (po_order_line.order_line_amnt) >= 1000 AND sum (po_order_line.order_line_amnt) < 2000
        THEN 'MEDIUM'
    WHEN sum (po_order_line.order_line_amnt) >= 2000 
        THEN 'LARGE'
    ELSE 
        sum (po_order_line.order_line_amnt)	
    END,
FROM
    popes:popesdba.po_order po_order,
    popes:popesdba.po_order_line po_order_line,
    popes:popesdba.po_invc po_invc
WHERE
    po_fi_cnrct.cnrct_id = po_order.fi_cnrct_id AND
    po_order.order_id = po_order_line.order_id AND
    po_order.order_stat_cd = po_order_line.order_stat_cd AND
    po_order.invc_id = po_invc.invc_id
)