Find a value of greater than or null in Mongo Collection

find.js

var coupon = Coupons.findOne({
			codes: { $in: [coupon_code] },
			valid: true,
			redeem_by: { $lte: now },
			$or: [
				{ end_date: { $gte: now } },
				{ end_date: null }
			]
		});