this.map = map;
    totalSize = 0;
    for (Collection<V> values : map.values()) {
      checkArgument(!values.isEmpty());
      totalSize += values.size();
    }
  }

  /**
   * Creates an unmodifiable, empty collection of values.
   *
   * <p>This is used in {@link #removeAll} on an empty key.
   */
  Collection<V> createUnmodifiableEmptyCollection() {
    return unmodifiableCollectionSubclass(createCollection());

    /* No aliasing on inverse operations. */
    assertSame(unmod.inverse(), unmod.inverse());
    assertSame(unmod, unmod.inverse().inverse());

    /* Unmodifiable is a view. */
    mod.put(4, "four");
    assertEquals(true, unmod.get(4).equals("four"));
    assertEquals(true, unmod.inverse().get("four").equals(4));

    /* No aliasing on inverse operations. */
    assertSame(unmod.inverse(), unmod.inverse());
    assertSame(unmod, unmod.inverse().inverse());

    /* Unmodifiable is a view. */
    mod.put(4, "four");
    assertEquals(true, unmod.get(4).equals("four"));
    assertEquals(true, unmod.inverse().get("four").equals(4));