rPi Node Red - Using prosody

[StrongTown]

Has anyone had luck using the speech rate adjustment parameter prosody?

When I create a string like " <prosody rate='slow'>This is two thirds normal speech rate <prosody rate='medium'>and this is normal speech." and then feed it to swift, swift just produces an error. If I remove the <content> in the string it works fine (except the speech rate doesn't change of course).

Any hints, ideas or suggestions?
TIA
ST