MKPolygon area calculation

I'm trying to make an area calculation category for MKPolygon. I found some JS code https://github.com/mapbox/geojson-area/blob/master/index.js#L1 with a link to the algorithm: http://trs-new.jpl.nasa.gov/dspace/handle/2014/40409. It says:

Here is my code, which gave a wrong result (thousands times more than actual):

#define kEarthRadius 6378137
@implementation MKPolygon (AreaCalculation)
- (double) area {
    double area = 0;
    NSArray *coords = [self coordinates];
    if (coords.count > 2) {
        CLLocationCoordinate2D p1, p2;
        for (int i = 0; i < coords.count - 1; i++) {
            p1 = [coords[i] MKCoordinateValue];
            p2 = [coords[i + 1] MKCoordinateValue];
            area += degreesToRadians(p2.longitude - p1.longitude) * (2 + sinf(degreesToRadians(p1.latitude)) + sinf(degreesToRadians(p2.latitude)));
        }

        area = area * kEarthRadius * kEarthRadius / 2;
    }

    return area;
}
- (NSArray *)coordinates {
    NSMutableArray *points = [NSMutableArray arrayWithCapacity:self.pointCount];
    for (int i = 0; i < self.pointCount; i++) {
        MKMapPoint *point = &self.points[i];
        [points addObject:[NSValue valueWithMKCoordinate:MKCoordinateForMapPoint(* point)]];
    }
    return points.copy;
}

double degreesToRadians(double radius) {
    return radius * M_PI / 180;
}

@end

What did I miss?

The final step for i = N-1 and i+1 = 0 (wrap around) is missing in your loop.