latin_ipa_forward checkpoint dropping german letter 'ß'

Hi,

i am using the latin_ipa_forward.pt checkpoint to phonemize large amounts of german text.while it is working fine in almost every case, for some reason, it seems to drop the german letter 'ß'. eg:

`>>>phonemizer('ich wohne in der sesamstraße.', lang='de')` 
_ɪç voːnə ʔɪn deːr zezamstʁaː._

any idea how to fix this?
Thanks!