@Test
public void words_nonnull_map_return_keyset() throws Exception {

    final Map<String, String[]> map = new HashMap<String, String[]>();
    map.put("1", null);
    PinyinMapDict dict = new PinyinMapDict() {
        @Override
        public Map<String, String[]> mapping() {
            return map;
        }
    };
    assertThat(dict.words(), is(map.keySet()));
}