segmentation fault while reading in text file ( c++ )

Question

I have my own little graph class and I want to create a graph by reading in a file, containing the necessary information like number of vertices, edges, etc. The files are in the STP format. The code works fine, if I just read in the file word by word.

Now I need to identify certain keywords like "Nodes", followed by the number of vertices. When reaching such a keyword, I read in the next word which I then store as the number of vertices. This results in a segmentation fault

Here's the code:

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>

#include "graph2.h"

Graph readin(char const * filename) {
    Graph mygraph(0);
    std::ifstream file (filename);
    if (not file) throw std::runtime_error("Couldn't open file.");
    std::string line, word;
    int head, tail;

    // magic password 
    std::getline(file,line);
    if (line == "33D32945 STP File, STP Format Version 1.0") { 
        while (file >> word) {
            if (word == "Nodes") {
                file >> mygraph.numvertices;
            }
            else if (word == "E") {
                file >> tail;
                file >> head;
                mygraph.edge(tail,head,Graph::add);
            }
            else if (word == "EOF") break;
        }
    }

    else throw std::runtime_error("File is not in STP file format.");

    return mygraph;
}


int main(int argc, char* argv[]) {
    if (argc > 1) {
        Graph mygraph = readin(argv[1]);
        mygraph.print();
    }
    return 0;
}


It works fine like that:

    if (line == "33D32945 STP File, STP Format Version 1.0") { 
        while (file >> word) {
            std::cout << word << std::endl;
        }
    }

Anybody got any idea?