C# Debug Error

I am getting this error ---- Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: Cannot perform

runtime binding on a null reference

at CallSite.Target(Closure, CallSite, Object)

at System.Dynamic.UpdateDelegates.UpdateAdExecute1[T0,TRET](CallSite, site, 

T0 arg0)

And the line of code that is causing me issues is this one --- it is supposed to be converting the spreadsheet to a pdf

oWB = oXL.ActiveWorkbook;
oXL.ActiveWorkbook.ExportAsFixedFormat(Type:Excel.XLFixedFormatType.xlTypePDF, Filename: "Test.pdf", Quality: XlFixedFormatQuality.xlQualityStandard, OpenAfterPublish: false);