Failure when using recurrence event

[razshlomo]

Failure after defining a recurrence event with the following error: <LoadError> <EntityId></EntityId> <Error>System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer&amp; number, NumberFormatInfo info, Boolean parseDecimal) at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at System.Convert.ToInt32(String value) at DDay.iCal.Serialization.iCalendar.RecurrencePatternSerializer.AddInt32Values(IList1 list, String value) at DDay.iCal.Serialization.iCalendar.RecurrencePatternSerializer.Deserialize(TextReader tr) at DDay.iCal.Serialization.iCalendar.DataMapSerializer.Deserialize(TextReader tr) at DDay.iCal.iCalParser.property(ISerializationContext ctx, ICalendarPropertyListContainer c) at DDay.iCal.iCalParser.component(ISerializationContext ctx, ISerializerFactory sf, ICalendarComponentFactory cf, ICalendarObject o) at DDay.iCal.iCalParser.component(ISerializationContext ctx, ISerializerFactory sf, ICalendarComponentFactory cf, ICalendarObject o) at DDay.iCal.iCalParser.icalbody(ISerializationContext ctx, IICalendar iCal) at DDay.iCal.iCalParser.icalendar(ISerializationContext ctx) at DDay.iCal.Serialization.iCalendar.iCalendarSerializer.Deserialize(TextReader tr) at CalDavSynchronizer.Implementation.CalDavRepository1.DeserializeCalendar(String iCalData, IStringSerializer calendarSerializer) at CalDavSynchronizer.Implementation.CalDavRepository1.TryDeserializeCalendar(String iCalData, IICalendar&amp; calendar, WebResourceName uriOfCalendarForLogging, IStringSerializer calendarSerializer, ILoadEntityLogger logger)</Error> <IsAEntity>false</IsAEntity> <IsWarning>false</IsWarning> </LoadError>

After investigation, I found the following line in the ics: RRULE:FREQ=YEARLY;BYMONTH=4;BYMONTHDAY=;BYDAY=-1;UNTIL=20120330T000000Z

It looks like that BYMONTHDAY is empty.