Error

hi,
How can I interpret this error, where do I look for the error?

[FormatException: Input string was not in a correct format.]
System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) +7471335
System.Number.ParseDouble(String value, NumberStyles options, NumberFormatInfo numfmt) +115
System.Double.Parse(String s, NumberStyles style, NumberFormatInfo info) +192
FastReport.Variant.ToDouble() +119
FastReport.Variant.d0B1XOKtY(Type ) +560
FastReport.Variant.8NnvYsFsL(Type , Boolean , IFormatProvider ) +84
FastReport.Variant.ToDouble(IFormatProvider formatProvider) +30
System.Convert.ToDouble(Object value) +28
FastReport.ReportScript.CalcExpression(String expression, Variant Value) +226

[TargetInvocationException: Exception has been thrown by the target of an invocation.]
System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) +0
System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) +71
System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) +261
System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) +29
System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters) +19
67TjyttklyEHhRuOGx.lqojRuXM9Mkt1iW5by.u7yburCWk(Object[] ) +180
cwC4EwnGMR2QXNJtHo.XccvDn8v3njxP77EWv.RGARmhJyj(String , Variant ) +116
FastReport.Report.CalcExpression(String expression, Variant value) +150
FastReport.Report.Calc(String expression, Variant value) +326
FastReport.Report.Calc(String expression) +39
FastReport.TextObject.GetData() +285
FastReport.BandBase.GetData() +136
FastReport.Engine.ReportEngine.mduySGRLP(BandBase , Boolean ) +40
FastReport.Engine.ReportEngine.Ptw56pn9R(BandBase , Boolean ) +112
FastReport.Engine.ReportEngine.YMypUDJNl(BandBase , Boolean ) +164
FastReport.Engine.ReportEngine.8X9XHbEby(GroupHeaderBand ) +80
FastReport.Engine.ReportEngine.PfcUOI2gb(YUoU0ILXkvFfVUb92TK ) +313
FastReport.Engine.ReportEngine.PfcUOI2gb(YUoU0ILXkvFfVUb92TK ) +247
FastReport.Engine.ReportEngine.PfcUOI2gb(YUoU0ILXkvFfVUb92TK ) +247
FastReport.Engine.ReportEngine.u5dcc1by5(GroupHeaderBand ) +48
FastReport.Engine.ReportEngine.u0cbEuQVC7(BandCollection ) +107
FastReport.Engine.ReportEngine.Zsnb3svMVc(ReportPage ) +149
FastReport.Engine.ReportEngine.R3KbH8IfMI() +85
FastReport.Engine.ReportEngine.HdBEuSauT(ReportPage ) +33
FastReport.Engine.ReportEngine.KNEMorep1(Boolean , Boolean , ReportPage ) +220
FastReport.Engine.ReportEngine.pxMJbiJPp(Boolean , Boolean ) +19
FastReport.Report.Prepare(Boolean append) +218
FastReport.Web.WebReport.s850bUrQZ() +353
FastReport.Web.WebReport.OnLoad(EventArgs e) +68
System.Web.UI.Control.LoadRecursive() +50
System.Web.UI.Control.LoadRecursive() +141
System.Web.UI.Control.LoadRecursive() +141
System.Web.UI.Control.LoadRecursive() +141
System.Web.UI.Control.LoadRecursive() +141
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627