MainWindow.h, defining the custom widget.

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtOpenGL>

class MainWindow : public QOpenGLWidget, protected QOpenGLFunctions
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);

private:
    QOpenGLVertexArrayObject m_vao;
    QOpenGLBuffer m_vbo;
    QOpenGLShaderProgram *m_program;

protected:
    void initializeGL();
    void paintGL();
};

#endif // MAINWINDOW_H

MainWindow.ccp

// Shader sources
const GLchar* vertexSource =
    "in vec2 position;"
    "void main()"
    "{"
    "    gl_Position = vec4(position, 0.0, 1.0);"
    "}";

const GLchar* fragmentSource =
    "void main()"
    "{"
    "   gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);"
    "}";


MainWindow::MainWindow(QWidget *parent) :
    QOpenGLWidget(parent),
    m_program(0)
{
    ;
}


void MainWindow::initializeGL()
{
    // Expose GL functions
    initializeOpenGLFunctions();

    // Set clear color
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

    // Instantiate program, link shaders, bind program
    m_program = new QOpenGLShaderProgram();
    m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexSource);
    m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentSource);
    m_program->link();
    m_program->bind();

    // Vertex array object
    m_vao.create();
    m_vao.bind();

    // Vertex buffer object
    m_vbo.create();
    m_vbo.bind();

    GLfloat vertices[] = {
         0.0f,  0.5f,
         0.5f, -0.5f,
        -0.5f, -0.5f
    };

    m_vbo.allocate(vertices, sizeof(vertices));

    // Specify the layout of the vertex data
    int posAttrib = m_program->attributeLocation("position");
    glEnableVertexAttribArray(posAttrib);
    glVertexAttribPointer(posAttrib, 2, GL_FLOAT, GL_FALSE, 0, 0);

    m_program->release();
}


void MainWindow::paintGL()
{
    // Clear the screen to black
    glClear(GL_COLOR_BUFFER_BIT);

    // Draw a triangle from the 3 vertices
    glDrawArrays(GL_TRIANGLES, 0, 3);
}

It draws the triangle as it should, but it's white instead of red. If I comment out the line where I compile the fragment shader, I get the same result.
Main loop just instantiates a QApplication and a MainWindow.
I've run the hellogl2 example project, which works regularly. Thanks in advance.