diff --git a/src/notifications/notifications.dto.ts b/src/notifications/notifications.dto.ts
index 995d586..1044321 100644
--- a/src/notifications/notifications.dto.ts
+++ b/src/notifications/notifications.dto.ts
@@ -1,4 +1,4 @@
-import { IsBoolean, IsIn, IsOptional, IsString, IsUrl, ValidateNested } from "class-validator";
+import { IsBoolean, IsIn, IsNumber, IsOptional, IsString, IsUrl, ValidateNested } from "class-validator";
 import { Type } from "class-transformer";
 
 export class UpdateNotificationPreferencesDto {
@@ -30,4 +30,11 @@ export class SavePushSubscriptionDto {
   @ValidateNested()
   @Type(() => PushKeysDto)
   keys!: PushKeysDto;
+
+  // Included by the browser's PushSubscription.toJSON() when the push
+  // service provides an expiry; not currently used server-side, but must be
+  // declared or the whitelist validation pipe rejects the whole request.
+  @IsOptional()
+  @IsNumber()
+  expirationTime?: number | null;
 }