PublicSuffixMatcher::getDomainRoot with expectation fails on invalid domain names.

Assume 
    PublicSuffixMatcher matcher = PublicSuffixMatcherLoader.getDefault();
Then this 
    matcher.getDomainRoot("www.example.unknown")
returns
    "unknown"
which is correct because no expectation was given about the domain name.

However when I do set a required expectation with something like
    matcher.getDomainRoot("www.example.unknown", DomainType.ICANN)
then the current implementation still returns
   "unknown"
instead of the expected 
   null
indicating that there was no match.