Unable to close line in purchase order by DI

HI all,

i tried to update purchaseOrder document after i try to close it row by it current command:

diPurchaseOrder.Lines.LinesStatus=di.BoStatus.bost_Close;

i got current message error:

the weird thing ia that i succed to close saleorder Linenum through di by the same logic but in purchase order it failed.

i will appreciate if someone can assist  me.