diff --git a/src/coupon-cell/index.tsx b/src/coupon-cell/index.tsx
index 87a9c1aa1..86e3b3d59 100644
--- a/src/coupon-cell/index.tsx
+++ b/src/coupon-cell/index.tsx
@@ -27,12 +27,13 @@ function formatValue(props: CouponCellProps) {
 
   if (coupon) {
     let value = 0;
+
     if (isDef(coupon.value)) {
       ({ value } = coupon);
     }
 
     if (isDef(coupon.denominations)) {
-      value = coupon.denominations!;
+      value = coupon.denominations;
     }
 
     return `-${currency} ${(value / 100).toFixed(2)}`;
diff --git a/src/utils/index.ts b/src/utils/index.ts
index 83f5ff38b..6386cc111 100644
--- a/src/utils/index.ts
+++ b/src/utils/index.ts
@@ -9,7 +9,7 @@ export const isServer: boolean = Vue.prototype.$isServer;
 // eslint-disable-next-line @typescript-eslint/no-empty-function
 export function noop() {}
 
-export function isDef(val: unknown): boolean {
+export function isDef<T>(val: T): val is NonNullable<T> {
   return val !== undefined && val !== null;
 }