Segmentation Fault 11, simple but fatal

Segmentation Fault 11, simple but fatal

I am getting a segmentation fault 11 in the following code. This code is
basically straight from the book, that is why it has be puzzled.
Similarity.h
#ifndef SIMILARITY_H
#define SIMILARITY_H
#include "../../Utilities/Scanner.h"
#include "../../Utilities/Utils.h"
class Similarity
{
public:
Similarity();
virtual ~Similarity();
void readData(Scanner& inStream);
string maxCosine();
string maxJaccard();
string toString();
private:
vector<string> theData;
};