Null Object Reference on SUM query in android

I am getting a null object reference error when I try to query my db for the sum of a column.

As far as I can tell the query code is correct.

Here is my query:

int tripSummary(){
    int distance;
    Cursor mCursor = mDb.rawQuery("SELECT SUM (distance) FROM Trip WHERE vehicle = 'Car';", null);

    if(mCursor.moveToFirst()) {
        distance = mCursor.getInt(0);
    }else{
        distance = -1;
    }
    return distance;
}

Here is the fragment that I call the query from:

    public class SummaryFragment extends Fragment {

TripsDbAdapter dbHelper;

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.summary_fragment, container, false);

    String carbonFootprint = "Your carbon footprint for all all trips by category is:";
    TextView footprint = (TextView) view.findViewById(R.id.carbonFootprint);
    footprint.setText(carbonFootprint);

    String carTrips = "Total distance for car = " + calculateFootprint();
    TextView car = (TextView) view.findViewById(R.id.car);
    car.setText(carTrips);

    return view;
}

public int calculateFootprint(){

    dbHelper.tripSummary();

    int distance = dbHelper.tripSummary();

    return distance;
}

And here is the logcat:

    E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.brianmcmahon.carbonfootprintcalculator, PID: 3914
java.lang.NullPointerException: Attempt to invoke virtual method 'int com.brianmcmahon.carbonfootprintcalculator.TripsDbAdapter.tripSummary()' on a null object reference
        at com.brianmcmahon.carbonfootprintcalculator.SummaryFragment.calculateFootprint(SummaryFragment.java:41)
        at com.brianmcmahon.carbonfootprintcalculator.SummaryFragment.onCreateView(SummaryFragment.java:32)
        at android.support.v4.app.Fragment.performCreateView(Fragment.java:1786)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:947)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
        at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
        at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:454)

Any suggestions would be appreciated.