update to VBAK table

hi Gurus ,

i am facing a very strange problem

i have update couple of z fields in master table VBAK i am writing a normal update statements

but it is not updating the value of that field and setting the sy-subrc to 4 . i am passing just the vbeln in the where clause for updating.

can anyone suggest me why my updating failing.

thanks in advance

regarsd,

khushi