When parsing some XLSX files you can find some empty cells when using AsDataset() method.
(sample file attached: resulting dataset doesn't contain data for column PezziA1).
 
This happens because you are trying to read unexistant data in ExcelOpenXmlReader.ReadSheetRow(XlsxWorksheet sheet)
Wrong (actual) code follows:

                    _xmlReader.ReadToDescendant(XlsxWorksheet.N_v); <-- ERROR!!! (the file contains "<c r=\"O2\" s=\"6\" xmlns=\"http://schemas.openxmlformats.org/spreadsheetml/2006/main\" />" so there's no descendant)
                    object o = _xmlReader.ReadElementContentAsObject();

 

                    if (null != a_t && a_t == XlsxWorksheet.A_s)
                    {
                        o = _workbook.SST[Convert.ToInt32(o)];
                    }
                    else if (null != a_s)
                    {
                        XlsxXf xf = _workbook.Styles.CellXfs[int.Parse(a_s)];

 

                        if (xf.ApplyNumberFormat && IsDateTimeStyle(xf.NumFmtId) && o != null && o.ToString() != string.Empty)
                        {
                            o = DateTime.FromOADate(Convert.ToDouble(o, CultureInfo.InvariantCulture));
                        }
                    }

 

                    if (col - 1 < _cellsValues.Length)
                        _cellsValues[col - 1] = o;

 
You have to change the code in this way for it to work properly:

                    if (_xmlReader.ReadToDescendant(XlsxWorksheet.N_v))
                    {
                        object o = _xmlReader.ReadElementContentAsObject();

 

                        if (null != a_t && a_t == XlsxWorksheet.A_s)
                        {
                            o = _workbook.SST[Convert.ToInt32(o)];
                        }
                        else if (null != a_s)
                        {
                            XlsxXf xf = _workbook.Styles.CellXfs[int.Parse(a_s)];

 

                            if (xf.ApplyNumberFormat && IsDateTimeStyle(xf.NumFmtId) && o != null && o.ToString() != string.Empty)
                            {
                                o = DateTime.FromOADate(Convert.ToDouble(o, CultureInfo.InvariantCulture));
                            }
                        }

 

                        if (col - 1 < _cellsValues.Length)
                            _cellsValues[col - 1] = o;
                    }

 
Hope this helps someone.
I saw lots of people complaining about missing data on XLSX, this will probably fix it.
If it does solve your problem please let me know.
Bye,
Lino Barreca