If you look at the docs of the HashMap#keySet() method, you’ll get your answer(emphasis mine).
Returns a Set view of the keys contained in this map. The set is
backed by the map, so changes to the map are reflected in the set, and
vice-versa. If the map is modified while an iteration over the set is
in progress (except through the iterator’s own remove operation), the
results of the iteration are undefined. The set supports element
removal, which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove, removeAll, retainAll, and clear
operations. It does not support the add or addAll operations.
Therefore, you need to create a new set and add all the elements to it, instead of adding the elements to the Set returned by the keySet().