.net core weboptimizer not creating a bundle

I'm trying to use https://github.com/ligershark/WebOptimizer as suggested by microsoft in order to bundle and minimise my js files, so following the instructions, I have the following in my startup.cs:

app.UseWebOptimizer();
app.UseStaticFiles();

and in my service configuration (startup.cs):

services.AddWebOptimizer(pipeline =>
{
    pipeline.AddJavaScriptBundle("/js/site.js",     // I have tried using MinifyJsFiles (without the use content root) instead of AddJavaScriptBundle
        "/lib/jquery-ui-1.13.1.custom/jquery-ui.js",
        "/js/auto-complete.js")
      .UseContentRoot();

    pipeline.MinifyJsFiles(); // I have tried with and without this line
});

and in my _layout:

<script src="~/js/site.js"></script>

But whenever I browse to the page, I'm just getting a 404 error in the network tab when it tries to load the site.js

Is there something I have missed? All the files are in their correct places in the wwwroot folder of the site