I've encountered an issue wherein I'm trying to purge a (third-party) CSS file and I'm getting the following error:

CSS Parser Error: probably have something funny in your CSS, change it then please try again.
Reason: missing '}'
Line: 157
Column: 1
Filename: demo/test1.css

There is no missing } in the line mentioned nor anywhere else in the file as far as I'm aware. The filename is also not demo/test1.css.

Here is my Gulp code:

const gulp = require('gulp');
const concat = require('gulp-concat');
const streamqueue = require('streamqueue');
const cssPurge = require('gulp-css-purge');

// Concatenate and minify stylesheets
gulp.task('styles', function() {
    // inspired by https://stackoverflow.com/a/23507836
    return streamqueue({ objectMode: true },
        gulp.src('./node_modules/purecss/build/pure.css'),
        gulp.src('./static/app.css'),
        gulp.src('./node_modules/codemirror/lib/codemirror.css'),
    )
    .pipe(concat('styles.css'))
    .pipe(cssPurge({
        trim : true,
        shorten : true,
        verbose : false
    }))
    .pipe(gulp.dest('./static'));
});

The problem is with the ./node_modules/codemirror/lib/codemirror.css file because when I comment this line I can purge the other two without issues. The problem is also not with concatenation or the use of streamqueue because even without it I get the same error.

Now, the following run from CLI works without any problem and produces a valid minified CSS file:

./node_modules/.bin/css-purge -i ./node_modules/codemirror/lib/codemirror.css

Therefore I assume there's a problem in this package and not in css-purge itself.