Error in RDLC REPORT in vs2010- An error occurred during local report processing, Object reference not set to an object instance

Question

i'm using rdlc in visual studio 2010 and i get this error during load rdlc in this part

_report.Render("PDF", deviceInfo, FunzioneDiCallBack, out W_warning);

i have understood that the problem is in this condition

  if (periodo >= dataDa && periodo <= dataA)
          W_ret_val.Add(W_rec);

where periodo is my data and should stay in that range.. but in this way i have this error

    InnerException  {"Riferimento a un oggetto non impostato su un'istanza di oggetto."}     
    System.Exception {System.NullReferenceException}

stacktrace:

in Microsoft.ReportingServices.Rendering.HPBProcessing.Tablix.PageStructStaticMemberCell.AddToPageCHContent(List1 rowHeights, ScalableList1 columnInfo, Int32 rowIndex, Int32 colIndex, Boolean isLTR, RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState, Boolean& hasLabels) in Microsoft.ReportingServices.Rendering.HPBProcessing.Tablix.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.PageItemContainer.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.SubReport.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.Tablix.RowInfo.AddToPageContent(ScalableList1 columnInfo, Int32& colsOnPage, Boolean isLTR, Boolean pinnedToParentCell, RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.Tablix.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.PageItemContainer.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.SubReport.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.PageItemContainer.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.PageItemContainer.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.SubReport.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.Tablix.RowInfo.AddToPageContent(ScalableList1 columnInfo, Int32& colsOnPage, Boolean isLTR, Boolean pinnedToParentCell, RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.Tablix.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.PageItemContainer.AddToPage(RPLWriter rplWriter, PageContext pageContext, Double pageLeft, Double pageTop, Double pageRight, Double pageBottom, RepeatState repeatState) in Microsoft.ReportingServices.Rendering.HPBProcessing.Report.NextPage(RPLWriter rplWriter, Int32 page, Int32 totalPages) in Microsoft.ReportingServices.Rendering.HPBProcessing.HPBProcessing.NextPage() in Microsoft.ReportingServices.Rendering.HPBProcessing.HPBProcessing.Init(Report report, PaginationSettings pagination, CreateAndRegisterStream createAndRegisterStream, Hashtable& renderProperties) in Microsoft.ReportingServices.Rendering.HPBProcessing.HPBProcessing..ctor(Report report, PaginationSettings pagination, CreateAndRegisterStream createAndRegisterStream, Hashtable& renderProperties) in Microsoft.ReportingServices.Rendering.ImageRenderer.PDFRenderer.Render(Report report, NameValueCollection deviceInfo, Hashtable renderProperties, CreateAndRegisterStream createAndRegisterStream) in Microsoft.ReportingServices.Rendering.ImageRenderer.RendererBase.Render(Report report, NameValueCollection reportServerParameters, NameValueCollection deviceInfo, NameValueCollection clientCapabilities, Hashtable& renderProperties, CreateAndRegisterStream createAndRegisterStream)

but if i use this condition

if (periodo >= dataDa && periodo < dataA)
          W_ret_val.Add(W_rec);

or this condition

if (periodo >= dataDa && periodo = dataA)
          W_ret_val.Add(W_rec);

it works fine.. and i don't undestrand why...