Adding lyrics: System.NotImplementedException: The method or operation is not implemented.

[Black-Platypus]

trying to add lyrics:

Id3Tag tag = mp3.GetTag(Id3TagFamily.Version2X);
tag.Lyrics.Add("foo", "bar");
mp3.WriteTag(tag, WriteConflictAction.Replace);

throws:

System.NotImplementedException: The method or operation is not implemented.
at Id3.v2.Id3V23Handler.EncodeLyrics(Id3Frame id3Frame)
at Id3.v2.Id3V23Handler.GetTagBytes(Id3Tag tag)
at Id3.v2.Id3V23Handler.WriteTag(Stream stream, Id3Tag tag)
at Id3.Mp3.WriteTag(Id3Tag tag, WriteConflictAction conflictAction)

, as does

Id3Tag tag = mp3.GetTag(Id3TagFamily.Version2X);
Id3.Frames.LyricsFrame lf = new Id3.Frames.LyricsFrame("foo", "bar");
lf.Language = Id3Language.ger;
lf.EncodingType = Id3TextEncoding.Unicode;
tag.Lyrics.Add(lf);
mp3.WriteTag(tag, WriteConflictAction.Replace);

I thought maybe setting language and encoding type might help, but no luck.
Am I doing something wrong, or is there something actually not implemented?