checkNotNull(keyFunction);
    while (values.hasNext()) {
      V value = values.next();
      builder.put(keyFunction.apply(value), value);
    }
    try {
      return builder.buildOrThrow();
    } catch (IllegalArgumentException duplicateKeys) {
      throw new IllegalArgumentException(
          duplicateKeys.getMessage()
              + ". To index multiple values under a key, use Multimaps.index.");

    checkNotNull(keyFunction);
    while (values.hasNext()) {
      V value = values.next();
      builder.put(keyFunction.apply(value), value);
    }
    try {
      return builder.buildOrThrow();
    } catch (IllegalArgumentException duplicateKeys) {
      throw new IllegalArgumentException(
          duplicateKeys.getMessage()
              + ". To index multiple values under a key, use Multimaps.index.");