GulpUglifyError:Unable to minify JavaScript

I am trying to minify my script files for which i am using gulp task runner And I am trying gulp-uglify plugin

Code:

 gulp.task('concat', function() {
    return gulp.src('app/**/*.js')
        // .pipe(concat('script.js'))
        .pipe(uglify())
        .pipe(gulp.dest('./dist/'))
});

but i am getting error as

when i try to run gulp task as gulp concat Any help would be appreciated