error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘std::ostream {aka std::basic_ostream<char>}’)

I am trying to build Tramonto fDFT package, but .cpp file got error like this: error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream}’ and ‘std::ostream {aka std::basic_ostream}’) stream << x1.leftshift(stream) << " " << x2.leftshift(stream)

Here is the code:

ostream& NOXLOCA::Tramonto::PTVector::leftshift(ostream& stream) const  
{
  stream << "{ ";
  stream <<  x1.leftshift(stream) << " " << x2.leftshift(stream)
         <<  " [ " << ptp << "] ";
  stream << "}";
  return stream; 
}

ostream& operator<<(ostream& stream, const NOXLOCA::Tramonto::PTVector& v)
{
  return v.leftshift(stream);
}

void NOXLOCA::Tramonto::PTVector::print(std::ostream& stream) const
{
  stream << *this << endl;
}