Getting Error in Rowcommand Event :Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index"

hi..getting error here int addid = Convert.ToInt32(gvImages.DataKeys[rowindex]["CategoryID"]); that "Error:Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index"
i was declared the commandArgument='<%# Container.DisplayIndex %>' like this..
Plz help me..urjent..
protected void gvImages_RowCommand(object sender, GridViewCommandEventArgs e)     {         int rowindex = Convert.ToInt32(e.CommandArgument);//Identifying the row         try         {   ...