Using own class as key in map causes segmentation fault

Hi,

I'm trying to use my own class as a map key. However something strange
seems to be happening to the map which causes my program to segfault.
I'm at a loss as to what this might be. Can anyone help?

The complete example below segfaults for me under g++ 4.1.2. Using a
debugger it seems that one of the MultiDimKey objects the map is
holding becomes corrupt.

#include <vector>
#include <map>
#include <string>
#include <iostream>

using namespace std;

class MultiDimKey {
public:

  MultiDimKey() : keys() {
  }

  MultiDimKey(const vector<string> keys_in) : keys(keys_in) {
  }

  bool operator<(const MultiDimKey &other) const {

    const vector<string> other_keys = other.keys;

    for(size_t n=0;n<keys.size();n++) {
      if(keys[n] < other_keys[n]) return true;
    }

    return false;
  }

  vector<string> keys;
};

int main() {

  map<MultiDimKey,int> test;

  vector<string> s1;
  s1.push_back("3");
  s1.push_back("6");
  MultiDimKey k1(s1);
  test[k1]++;

  vector<string> s2;
  s2.push_back("5");
  s2.push_back("3");
  MultiDimKey k2(s2);
  test[k2]++;

  vector<string> s3;
  s3.push_back("6");
  s3.push_back("2");
  MultiDimKey k3(s3);
  test[k3]++;

  vector<string> s4;
  s4.push_back("1");
  s4.push_back("2");
  MultiDimKey k4(s4);
  test[k4]++;
}