Strings with Templates not working?

Hello,

I'm using a template to simulate a LinkedList from Java.It works without
problems, but when I want to use strings in main.cpp instead of char*, I
get the following error message:

$ ./Main
terminate called after throwing an instance of 'std::logic_error'
   what(): basic_string::_S_construct NULL not valid

I tried to import <string> both in main.cpp and listT.h, but the
errormsg was the same.

The classes look like this:

MAIN.CPP------------------------------------------
#include "ListT.h"
#include <iostream>

using namespace std;

int main(void) {

    ListT<string> *testList = new ListT<string>();
    testList->add("Test1");
    testList->add("Test2");
    testList->add("Test3");
    cout << testList->get(0) <<endl;
    cout << testList->get(1) <<endl;
    cout << testList->get(2) <<endl;
    delete testList;

return 0;
}
------------------------------------------------

LISTT.H-----------------------------------------

template <class T> class ListT {

    private:
     struct TKnoten {
         T daten;
         int index;
         TKnoten *next;
     };
     int counter;
     TKnoten *kopf;

    public:
       ListT();
       virtual ~ListT();
       void add(T element);
       //void remove(T element);
       T get(int i);
};

template <class T>
ListT<T>::ListT() {
     counter = 0;
     kopf = new TKnoten();
     kopf->next = 0;
}

template <class T>
ListT<T>::~ListT() {
     while (kopf->next != 0) {
         TKnoten *previous = new TKnoten();
         previous = kopf;
         kopf = kopf->next;
         delete previous;
     }
     delete kopf;
}

template <class T>
void ListT<T>::add(T element) {
     TKnoten *newKnoten = new TKnoten();

     newKnoten->next = kopf;
     kopf = newKnoten;
     kopf->daten = element;
     kopf->index = counter;
     counter++;
}

template <class T>
T ListT<T>::get(int i) {
     TKnoten *iterator = new TKnoten();
     T daten = 0;
     iterator = kopf;
     while (iterator->next != 0) {
          if (iterator->index == i) {
             daten = iterator->daten;
          }
          iterator = iterator->next;
     }
     delete iterator;
     return daten;
}

What's wrong with string as type? I'm using Linux with gcc (g++),

Thanks for your answers,

Markus