diff --git a/app/code/core/Mage/Sales/Model/Quote.php b/app/code/core/Mage/Sales/Model/Quote.php
index ceca03d598b..c051774cdb7 100644
--- a/app/code/core/Mage/Sales/Model/Quote.php
+++ b/app/code/core/Mage/Sales/Model/Quote.php
@@ -2096,10 +2096,10 @@ public function getCouponCode(): string
     }
 
     /**
-     * @param string $couponCode
+     * @param string|null $couponCode
      * @return $this
      */
-    public function setCouponCode(string $couponCode)
+    public function setCouponCode(?string $couponCode)
     {
         return $this->setData('coupon_code', $couponCode);
     }