LibGDX Model not rendering

I've been trying to learn libgdx's 3D functionality, because I want to make a turn-based game set on a 3D hexagonally tiled sphere.

As a simple starter project, I decided to try to create a simple icosahedron (in the future, I plan to subdivide it to create my map). This is my Java code:

package com.origin.hexasphere.screens;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Screen;
import com.badlogic.gdx.graphics.*;
import com.badlogic.gdx.graphics.g3d.Model;
import com.badlogic.gdx.graphics.g3d.ModelBatch;
import com.badlogic.gdx.graphics.g3d.ModelInstance;
import com.badlogic.gdx.graphics.glutils.ShaderProgram;
import com.badlogic.gdx.math.DelaunayTriangulator;

/** First screen of the application. Displayed after the application is created. */
public class GameScreen implements Screen
{
    public float phi = (float)(1 + Math.sqrt(5d))/2f;
    public float[] vertices = new float[]
    {
        0, 1, phi,
        0, -1, phi,
        0, 1, -phi,
        0, -1, -phi,
        phi, 0, 1,
        -phi, 0, 1,
        phi, 0, -1,
        -phi, 0, -1,
        1, phi, 0,
        -1, phi, 0,
        1, -phi, 0,
        -1, -phi, 0
    };
    private Mesh mesh;
    private Model model;
    private ShaderProgram shader;
    private ModelBatch batch;
    private PerspectiveCamera cam;
    private ModelInstance modelInstance;
    @Override
    public void show()
    {
        DelaunayTriangulator triangulator = new DelaunayTriangulator();
        this.cam = new PerspectiveCamera();
        cam.position.set(20f, 20f, 20f);
        cam.lookAt(0f, 0f, 0f);

        this.batch = new ModelBatch();

        this.shader = new ShaderProgram(
        Gdx.files.internal("shaders/vertex.glsl"),
        Gdx.files.internal("shaders/fragment.glsl"));

        short[] indices = triangulator.computeTriangles(vertices, true).toArray();
        this.mesh = new Mesh(false, vertices.length, indices.length,
            new VertexAttribute(VertexAttributes.Usage.Position, 3, ShaderProgram.POSITION_ATTRIBUTE ));
        this.mesh.setVertices(vertices);
        this.mesh.setIndices(indices);

        model = new Model();
        model.meshes.add(mesh);
        modelInstance = new ModelInstance(model);
    }

    @Override
    public void render(float delta)
    {
        cam.update();
        batch.begin(cam);
        batch.render(modelInstance);
        batch.end();

    }
}

This is my fragment shader:

void main()
{
    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
}

And this is my vertex shader:

attribute vec4 a_position;

uniform mat4 u_projectionViewMatrix;
void main()
{
    gl_Position =  u_projectionViewMatrix * a_position;
}

(I don't actually understand much about shaders outside of the very basics, so I'm trying to use the most minimal shaders I can find to test around with before I move onto something more complex).

My output is a completely black screen.