Created Unassigned: .save() method aborting with ArgumentNullException [9327]

This app has been working fine for many weeks, updating the same file, but something has apparently confusing ClosedXML. I've tried both versions 0.69.2 and 0.71.2.

System.ArgumentNullException was unhandled
HResult=-2147467261
Message=Value cannot be null.
Parameter name: source
Source=System.Core
ParamName=source
StackTrace:
at System.Linq.Enumerable.Contains[TSource](IEnumerable`1 source, TSource value, IEqualityComparer`1 comparer)
at System.Linq.Enumerable.Contains[TSource](IEnumerable`1 source, TSource value)
at ClosedXML.Excel.XLHyperlink.get_InternalAddress()
at ClosedXML.Excel.XLWorkbook.GenerateWorksheetPartContent(WorksheetPart worksheetPart, XLWorksheet xlWorksheet, SaveContext context)
at ClosedXML.Excel.XLWorkbook.CreateParts(SpreadsheetDocument document)
at ClosedXML.Excel.XLWorkbook.CreatePackage(String filePath)
at ClosedXML.Excel.XLWorkbook.Save()