non-working LÖVE app with a rotating quad that never draws

main.lua

local checkerboard
local shader

function love.load()
    love.graphics.setDefaultFilter('nearest', 'nearest')

    local imagedata = love.image.newImageData(8, 8)
    imagedata:mapPixel(function(x, y, r, g, b, a)
        if (x + y) % 2 == 1 then
            return 192, 192, 192, 255
        else
            return 64, 64, 64, 255
        end
    end)
    checkerboard = love.graphics.newImage(imagedata)

    shader = love.graphics.newShader([[
        vec4 effect( vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords )
        {
            vec4 texcolor = Texel(texture, texture_coords);
            return texcolor * color;
        }
    ]], [[
        extern float theta;

        vec4 position( mat4 transform_projection, vec4 vertex_position )
        {
            float l = -1024.0;
            float r = 1024.0;
            float t = 1024.0;
            float b = -1024.0;
            float n = 1024.0;
            float f = -1024.0;
            mat4 persp = mat4(
                2 * n / (r - l), 0, 0, 0,
                0, 2 * n / (t - b), 0, 0,
                (r + l) / (r - l), (t + b) / (t - b), (n + f) / (n - f), -1,
                0, 0, 2 * f * n / (n - f), 0);

            float c = cos(theta);
            float s = sin(theta);
            mat4 rot = mat4(
                c, 0, -s, 0,
                0, 1, 0, 0,
                s, 0, c, 0,
                0, 0, 0, 1);
            return transform_projection * persp * rot * vertex_position;
        }
    ]])

end

local theta = 0
function love.update(dt)
    theta = (theta + dt) % (math.pi * 2)
end

function love.draw()
    local w, h = love.graphics.getDimensions()
    local s = w / 2 * 0.9
    shader:send('theta', theta)
    love.graphics.setShader(shader)
    love.graphics.draw(checkerboard, 0, 0, 0, math.min(w, h) / 8)
    love.graphics.setShader()
end