Spark Coupon code set automatically to blank as default

I'm currently in the process of implementing Spark promotions within my application upon user signup. Having read the documentation I have the following 2 pieces of code: Spark::promotion('Coupon'); in my SparkServiceProvider

and the following

public static function promotion($coupon = null) {

    if (is_null($coupon)) {
        return static::$promotion;
    } else {
        static::$promotion = $coupon;
    }
}

within src/Configuration/ManagesAvailablePlans.php.

The code works as expected and if a valid coupon is entered then the discount is applied and a user is created.

However currently when the sign up page is loaded the word coupon is set as default within input box, ideally I would like the default to be blank. So far I have tried Spark::promotion(''); and Spark::promotion(null); however neither of these produce the desired effect, instead the input box just disappears.

Is there a way for the Spark::promotion section to be set as default blank?