The spec on the method Temporal plus(long amountToAdd, TemporalUnit unit) says:

http://download.java.net/jdk9/docs/api/java/time/temporal/Temporal.html#plus-long-java.time.temporal.TemporalUnit-

that UnsupportedTemporalTypeException is expected in the case if the unit is not supported

The following test shows that exception can be thrown not only in the case mentioned above:

public class Test {

    public static void main(String[] args) {
        Temporal temporal = JapaneseDate.now();
        if (IsoFields.WEEK_BASED_YEARS.isSupportedBy(temporal)) {
            try {
                temporal.minus(1, IsoFields.WEEK_BASED_YEARS);
            } catch (UnsupportedTemporalTypeException ute) {
                System.out.println("We don't expect UnsupportedTemporalTypeException since the temporal unit is supported by the temporal");
            } catch (Exception ex) {
                // other exceptions is OK
            }
        }
    }
}

Tests belonging to the JCK9 suite (not yet integrated) will fail due to this issue:

 - api/java_time/temporal/TemporalUnit/index.html#IsSupported