Map<? extends Class<? extends Number>, Number> nullValue = singletonMap(Number.class, null);
    assertThrows(NullPointerException.class, () -> ImmutableClassToInstanceMap.copyOf(nullValue));
  }

  public void testCopyOf_imap_empty() {
    Map<Class<?>, Object> in = emptyMap();
    ClassToInstanceMap<Object> map = ImmutableClassToInstanceMap.copyOf(in);
    assertTrue(map.isEmpty());
  }

  public void testCopyOf_imap_valid() {