--- old/src/java.base/share/classes/java/time/format/DateTimeFormatter.java	2016-09-15 14:53:56.170016472 +0000
+++ new/src/java.base/share/classes/java/time/format/DateTimeFormatter.java	2016-09-15 14:53:56.066033565 +0000
@@ -1685,6 +1685,8 @@
     public DateTimeFormatter withResolverFields(TemporalField... resolverFields) {
         Set<TemporalField> fields = null;
         if (resolverFields != null) {
+            // An unmodifiable set wrapper is used instead of Set.of because resolverFields may
+            // contain null and Set.of is null-hostile.
             fields = Collections.unmodifiableSet(new HashSet<>(Arrays.asList(resolverFields)));
         }
         if (Objects.equals(this.resolverFields, fields)) {