Rendering a simple rectangle with OpenGL 3 in D language

I'm going insane because I can't make a simple set of triangles appear in my screen.

I'm using OpenGL3 (without the deprecated fixed pipeline) using the derelict bindings for the D programming language.

Can you spot the error in the following program? It compiles just fine and doesn't throw any OpenGL/GLSL error. It just shows a blank screen with the clear color I set.

import std.string;
import std.conv;
import derelict.opengl3.gl3;
import derelict.sdl2.sdl2;

immutable string minimalVertexShader = `
#version 120
attribute vec2 position;
void main(void)
{
    gl_Position = vec4(position, 0, 1);
}
`;

immutable string minimalFragmentShader = `
#version 120
void main(void)
{
    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
}
`;

void main() {
    DerelictSDL2.load();
    DerelictGL3.load();

    if (SDL_Init(SDL_INIT_VIDEO) < 0) {
        throw new Exception("Failed to initialize SDL: " ~ to!string(SDL_GetError()));
    }

    // Set OpenGL version
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);

    // Set OpenGL attributes
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);

    auto sdlwindow = SDL_CreateWindow("D App",
        SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
        640, 480, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);

    if (!sdlwindow)
        throw new Exception("Failed to create a SDL window: " ~ to!string(SDL_GetError()));

    SDL_GL_CreateContext(sdlwindow);
    DerelictGL3.reload();

    float[] vertices = [ -1, -1,  1, -1,  -1, 1,  1, 1];
    ushort[] indices = [0, 1, 2, 3];
    uint vbo, ibo;
    // Create VBO
    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, vertices.sizeof, vertices.ptr, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // Create IBO
    glGenBuffers(1, &ibo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);