c++ operator overloading: NTL library export/import of a ZZ_pX vector

Question

I'm trying to export and import a NTL vector from type ZZ_pX. After Reading the source code i found this interesting operator functions:

istream& operator>>(istream& s, ZZ_pX& x)
{
   NTL_INPUT_CHECK_RET(s, s >> x.rep);
   x.normalize();
   return s;
}

ostream& operator<<(ostream& s, const ZZ_pX& a)
{
   return s << a.rep;
}

then i wrote this code for exporting :

    ZZ_pX phi;
    ofstream myfile;
    myfile.open ("phi.txt");
    myfile <<  phi;
    myfile.close();

and for the importing i tried to do something like this :

   ZZ_pX phi;
   std::ifstream dataFile("phi.txt");
   while (!dataFile.fail() && !dataFile.eof() )
   {
       dataFile >> phi ;
       cout << phi;
   }

The exporting is working but not the importing, error:

ZZ_p constructor called while modulus undefined
Aborted (core dumped)

Sorry if this is very simple as I quite new to C++