關於游標的定義

declare adm cursor
for select * from tableone order by cid
for update of name
open adm
   declare @a int,@b int
  set @a=1
 select @b=count(*) from tableone
 fetch next from adm
 while @@fetch_status=0
  begin
     update tableone set name=@a where current of adm
    set @a=@a+1
fetch next from adm
 end
 close adm
 deallocate adm


上述定義了一個更新游標，但是。執行後，提示:

Server: Msg 16957, Level 16, State 4, Line 2
FOR UPDATE cannot be specified on a READ ONLY cursor.


請問是什麼原因呢？？