Why does ConcurrentHashMap prevent null keys and values?

From the author of ConcurrentHashMap himself (Doug Lea):

The main reason that nulls aren’t allowed in ConcurrentMaps
(ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that
may be just barely tolerable in non-concurrent maps can’t be
accommodated. The main one is that if map.get(key) returns null, you
can’t detect whether the key explicitly maps to null vs the key isn’t
mapped. In a non-concurrent map, you can check this via
map.contains(key), but in a concurrent one, the map might have changed
between calls.