Error message using model parameter in Translate API

Question

I tried to use the Cloud Translation API Premium Beta edition. I build a small sample java application using the parameter model

( 
...
TranslateOption model = TranslateOption.model("nmt");
Translation translation = translate.translate(text, srcLang, tgtLang, model);
...
). 

I get an error message

Exception in thread "main" com.google.cloud.translate.TranslateException: Invalid JSON payload received. Unknown name "model": Cannot bind 'nmt'

Does anyone know, what's wrong?