private static @Nullable Object generateDummyArg(Parameter param, FreshValueGenerator generator)
      throws ParameterNotInstantiableException {
    if (isNullable(param)) {
      return null;
    }
    Object arg = generator.generateFresh(param.getType());
    if (arg == null) {
      throw new ParameterNotInstantiableException(param);
    }
    return arg;
  }