src/java.base/share/classes/java/util/concurrent/ConcurrentSkipListMap.java

@@ -1127,10 +1127,11 @@
                 (ConcurrentSkipListMap<K,V>) super.clone();
             clone.keySet = null;
             clone.entrySet = null;
             clone.values = null;
             clone.descendingMap = null;
+            clone.adder = null;
             clone.buildFromSorted(this);
             return clone;
         } catch (CloneNotSupportedException e) {
             throw new InternalError();
         }