Unexpected NullPointerException from Collectors.toMap

ADDITIONAL SYSTEM INFORMATION :
java.lang.NullPointerException

A DESCRIPTION OF THE PROBLEM :
List<Object[]> list = new ArrayList<>();

Object[] ar = {123,null};
Object[] arr = {321,null};
list.add(ar);
list.add(arr);

Map<Integer, String> collect = list.stream()
.filter(Objects::nonNull)
.collect(Collectors.toMap(obj -> (Integer)obj[0], obj -> (String)obj[1]));

System.out.println(collect);