Curly braces in glob without commas do not work

When curly braces ({}) are used in a glob, it does not work fine in case it contains no commas.

Here you have a testing example:

const { src, dest } = require('gulp'),
	extensions = ['css', 'js'];

exports.default = function () {
	return src(`src/**/*.{${extensions.join()}}`)
		.pipe(dest('dest/'));
};

It works good like this.

The point is that the trouble appears as soon as the array has only one item. To resolve it, it is necessary to add a trailing comma:

const { src, dest } = require('gulp'),
-	extensions = ['css', 'js'];
+	extensions = ['css'];

exports.default = function () {
-	return src(`src/**/*.{${extensions.join()}}`)
+	return src(`src/**/*.{${extensions.join()},}`)
		.pipe(dest('dest/'));
};

Why, with a list of options, some comma is always required? Is it right? Is this an expected feature? And Gulp cannot avoid it from usual operation of Bash, globbing patterns...? I did not found any documentation that explains this.