Select in LOOP error in PostgreSQL

Trying to start work following function, with SELECT in LOOP statement for other table records:

CREATE OR REPLACE FUNCTION parts."clearExpiredReserves"()
  RETURNS void AS
$BODY$DECLARE
    reserved_parts CURSOR FOR SELECT id, part_id, quantity FROM parts.operations_reg WHERE operation_type = 300
        AND operation_date < CURRENT_TIMESTAMP - interval '72 hours';

    spareparts_qty INT;
BEGIN
    FOR reserve_record IN reserved_parts LOOP
        SELECT p.quantity FROM parts.spareparts p WHERE p.id = reserve_record.part_id INTO spareparts_qty;

        IF spareparts_qty = reserved_parts.quantity THEN
            UPDATE parts.spareparts SET is_reserved = FALSE;
        END IF;

        DELETE FROM parts.operations_reg WHERE id = reserved_parts.id;
    END LOOP;

END;$BODY$
  LANGUAGE plpgsql VOLATILE SECURITY DEFINER
  COST 100;

And I have the error:

ERROR:  missing FROM-clause entry for table "reserved_parts"
LINE 1: SELECT spareparts_qty = reserved_parts.quantity
                                ^
QUERY:  SELECT spareparts_qty = reserved_parts.quantity
CONTEXT:  PL/pgSQL function "clearExpiredReserves" line 10 at IF

Where I was wrong?

Thanks.