glsl shader compiler ignores .length() access and removes the variable

love_shader_array_length_test.lua

local shader
local shader_code = [[
extern vec4 col[2];
vec4 effect(vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords)
{
    //float dummy = col[0][0];
    return vec4(1/col.length(),0,1,1);
}
]]

function love.load(arg)
    shader = love.graphics.newShader(shader_code)
end

function love.draw()
    love.graphics.setShader(shader)
    shader:send('col',{1,0,0,1},{1,0,0,1})
    love.graphics.rectangle('fill',0,0,400,400)
end