() -> ImmutableMap.ofEntries((Entry<Integer, Integer>) nullKey));
    Entry<@Nullable Integer, @Nullable Integer> nullValue = entry(23, null);
    assertThrows(
        NullPointerException.class,
        () -> ImmutableMap.ofEntries((Entry<Integer, Integer>) nullValue));
  }

  private static <T> Map<T, T> map(T... keysAndValues) {
    assertThat(keysAndValues.length % 2).isEqualTo(0);

        sourceMap.put("string", "test");
        sourceMap.put("integer", 42);
        sourceMap.put("long", 100L);
        sourceMap.put("double", 3.14);
        sourceMap.put("boolean", false);
        sourceMap.put("null_value", null);
        sourceMap.put("nested", nestedMap);

        event.setSourceMap(sourceMap);

        Map<String, Object> result = event.toSource();
        assertEquals("test", result.get("string"));

    boolean nullKeys = inverseFeatures.remove(MapFeature.ALLOWS_NULL_KEYS);
    boolean nullValues = inverseFeatures.remove(MapFeature.ALLOWS_NULL_VALUES);

    if (nullKeys) {
      inverseFeatures.add(MapFeature.ALLOWS_NULL_VALUES);
    }
    if (nullValues) {
      inverseFeatures.add(MapFeature.ALLOWS_NULL_KEYS);
    }

    inverseFeatures.add(NoRecurse.INVERSE);