Giving System.FormatException: String was not recognized as a valid DateTime. using datetime.ParseExact in C#

if (!string.IsNullOrEmpty(SessionDictionary.GetValue("UserDetails", "ExpiryDate")))
{
    DateTime ExpiryDate = DateTime.ParseExact(SessionDictionary.GetValue("UserDetails", "ExpiryDate"), "dd mmm yy", null);
    strDate = sitedata.FormatDate(ExpiryDate, TridionDateFormat.ShortDate);
}
else
{
    strDate = "-";
}

DateTime.ParseExact(..., "dd/MM/yyyy HH:mm:ss", CultureInfo.InvariantCulture);

DateTime ExpiryDate = DateTime.ParseExact(SessionDictionary.GetValue("UserDetails", "ExpiryDate"), "dd/MM/yyyy hh:mm:ss", null)

DateTime ExpiryDate = DateTime.ParseExact(SessionDictionary.GetValue("UserDetails", "ExpiryDate"), "G", null)