gridview in updatepanel

Hi:

I meet a problem about gridview in updatepanel. The problem is when I press the link button in gridview, 

the gridview in updatepanel will not invoke postback. I have try the following method:

protected void OrderGrid_RowDataBound(object sender, GridViewRowEventArgs e)
{
   LinkButton lb = e.Row.FindControl("MarkAsCompleteButton") as LinkButton;
   ScriptManager.GetCurrent(this).RegisterAsyncPostBackControl(lb);
}

However, it is not working. the lb will always be null. Is that any good way to fix this problem in updatepanel, Thanks a lot.