assertNull(docMap.get("nullKey"));

        assertNull(docMap.put("nullKey", "newValue"));
        assertEquals("newValue", docMap.get("nullKey"));
    }

    @Test
    public void test_map_interface_compliance() {
        Map<String, Object> parentMap = new HashMap<>();
        DocMap docMap = new DocMap(parentMap);

        assertTrue(docMap instanceof Map);
        assertTrue(docMap instanceof Map<?, ?>);