error expected initializer before 'namespace', c++

"vectortest.h" is a class representing a vector with its methods. So, the question is why the error " expected initializer before 'namespace' " occurs?

#ifndef LISTTEST_INCLUDED
#define LISTTEST_INCLUDED   1

#include <list> 
#include <string> 
#include <fstream>
#include <iostream>
#include "vectortest.h"
namespace listtest
{

   std::list<std::string> readfile( std::istream& input );


   std::list<std::string> randomstrings( size_t nr, size_t s );
      // Return nr random strings of length s.

   void sort_assign( std::list< std::string > & v );

   void sort_move( std::list< std::string > & v );
}

std::list<std::string> convToList(std::vector<std::string> &v);

std::ostream& 
operator << ( std::ostream& , const std::list< std::string > & );

#endif