Read Excel attachment - Error

string documentBody = data.ReadExcelFile(id);
byte[] byteArray = Encoding.UTF8.GetBytes(documentBody);

Stream stream = new MemoryStream(byteArray);

using (SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.Open(stream, false)) <-- ERROR LINE
{
   WorkbookPart workbookPart = spreadsheetDocument.WorkbookPart;