grid view rowcommand problem on gv_rowcommand

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ASP.NET

Hi
I am new asp.net user and I use gridview id gv
in gv I use event

protected void gv_RowCommand(object sender, GridVieCommandEventsArgs e)
{
    ViewState["info_id"]=e.CommandArgument;
    if(e.CommandName=="edit")
   {
      //do some work;
   }
}

and I given button in gridview and buttons command name will be set edit.
its shows error on this command i want to open a panel and edit the value as e.commandArgument.
please help me.
Thanks