Minification failed. Returning unminified contents

I resolved the problem bundling bootstrap.css by doing 2 things:

1. Include the bootstrap.css first in the bundle. The code sample in the question already does this, but I was not.
2. Add the official minified version (bootstrap.min.css) to the project in the same directory as the unminified version. This prompts the bundler to use the existing minified file instead of trying (and failing) to minify bootstrap.css itself. See the green arrow in the screenshot below.

Note that if you are using a specific theme, substitute bootstrap.css and bootstrap.min.css with the files provided by the theme. Here’s the working code from my project that uses the spacelab theme:

            bundles.Add(new StyleBundle(GetStyleBundlePath("bootstrap")).Include(
            "~/Content/3rdParty/bootstrap.spacelab.css",
            "~/Content/3rdParty/bootstrap-datepicker.css",
            "~/Content/3rdParty/bootstrap-multiselect.css"));