The code is as follows:

private static ValueEval EvaluateInternal(ValueEval arg, ValueEval iferror, int srcCellRow, int srcCellCol)
        {
            arg = WorkbookEvaluator.DereferenceResult(arg, srcCellRow, srcCellCol);
            if (arg is ErrorEval)
            {
               return iferror;
            }
            else
            {
                return arg;
            }
        }

NOPI programmers complain that the above code does not handle missing arguments:

https://github.com/nissl-lab/npoi/commit/fa5aab30649ac3344dad34216062cf4dac49efeb

Their fixed code is as follows:

private static ValueEval EvaluateInternal(ValueEval arg, ValueEval iferror, int...

 if (arg is ErrorEval)
            {
                return iferror;
                //if the 2nd argument is missing, use an empty string as default
                if (iferror is MissingArgEval)
                {
                    return new StringEval(string.Empty);
                }
                else
                {
                    return iferror;
                }
            }...
}