Shader program error with OpenGL 2.1

[gendx]

I wanted to try piston_window on a computer that only supports OpenGL 2.1. I adapted the hello world code to use OpenGL 2.1, but I get the following error.

thread 'main' panicked at 'Error InvalidValue creating program: Simple(VertexShader(Shader(1)), PixelShader(Shader(2)))', /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/gfx_device_gl-0.14.6/src/factory.rs:233:12

main.rs

extern crate piston_window;
extern crate sdl2_window;

use piston_window::*;
use sdl2_window::Sdl2Window;

fn main() {
    let mut window: PistonWindow<Sdl2Window> =
        WindowSettings::new("Test", [500; 500])
            .opengl(OpenGL::V2_1)
            .build()
            .unwrap();
    println!("Hello world!");

    while let Some(e) = window.next() {
        window.draw_2d(&e, |c, g| {
            clear([0.5, 0.5, 0.5, 1.0], g);
        });
    }
}

Cargo.toml

[dependencies]
piston_window = "0.74.0"
pistoncore-sdl2_window = "0.47.0"