How to cancel drop ship sales order line related with canceled po line

Hi,

We have issue that Drop ship order cannot be canelled in following case.
Please let me know how this issue can be resolved?

1. Created drop ship sales order with several lines.
2. Run 'Requisition Import' and purchase order was created.
3. Approved that purchase order.
4. Cancelled a line of that purchase order.
5. Tried to cancel sales order line cancel, but could not because of encounting following message,
    > "Order Line has associated Purchase Orders Approved"
6. Remove constraints acording to 292184.1
7. Tried to cancel again. But another message was shown and cannot cancel it.
    > Document #nnn:Cancel action at LINE_LOCATION Level is invalid for this document.(ACTION=CANCEL)