new Builder<String, Integer>().put(Maps.immutableEntry("one", 1)).build();
    assertEquals(Arrays.asList(1), multimap.get("one"));
  }

  public void testBuilder_withImmutableEntryAndNullContents() {
    Builder<String, Integer> builder = new Builder<>();
    assertThrows(
        NullPointerException.class, () -> builder.put(Maps.immutableEntry("one", (Integer) null)));
    assertThrows(