Blue part of pixel not updating

I am trying to do pixel point processes where I iteratively add some number to every component of every pixel of an image. I can update the red and green components of every pixel but for some reason this fails for the blue component. Here is my ofApp.cpp:

#include "ofApp.h"

//--------------------------------------------------------------
void ofApp::setup(){
	img.load("lineflower.png");
	W = ofGetWidth();
	H = ofGetHeight();
	img.resize(W, H);
	src = img.getPixels().getData();
}

//--------------------------------------------------------------
void ofApp::update(){
	for (int x = 0; x < W; x++) {
		for (int y = 0; y < H; y++) {
			int idx = 3 * (y * W + x);
			src[idx+2] += 4;
		}
	}
	img.update();
}

//--------------------------------------------------------------
void ofApp::draw(){
	img.draw(0, 0, W, H);
}

When I replace idx + 2 with idx or idx + 1 I get a constantly changing visual, but when I use idx + 2 I get a still image. It seems like for some reason it cannot access the blue components of pixels?

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