diff --git a/packages/backend/src/core/WebhookService.ts b/packages/backend/src/core/WebhookService.ts
index 30caa9682c..ac1e413de6 100644
--- a/packages/backend/src/core/WebhookService.ts
+++ b/packages/backend/src/core/WebhookService.ts
@@ -47,6 +47,7 @@ export class WebhookService implements OnApplicationShutdown {
 						this.webhooks.push({
 							...body,
 							createdAt: new Date(body.createdAt),
+							latestSentAt: body.latestSentAt ? new Date(body.latestSentAt) : null,
 						});
 					}
 					break;
@@ -57,11 +58,13 @@ export class WebhookService implements OnApplicationShutdown {
 							this.webhooks[i] = {
 								...body,
 								createdAt: new Date(body.createdAt),
+								latestSentAt: body.latestSentAt ? new Date(body.latestSentAt) : null,
 							};
 						} else {
 							this.webhooks.push({
 								...body,
 								createdAt: new Date(body.createdAt),
+								latestSentAt: body.latestSentAt ? new Date(body.latestSentAt) : null,
 							});
 						}
 					} else {
diff --git a/packages/backend/src/server/api/stream/types.ts b/packages/backend/src/server/api/stream/types.ts
index 9287952cb6..c450773055 100644
--- a/packages/backend/src/server/api/stream/types.ts
+++ b/packages/backend/src/server/api/stream/types.ts
@@ -178,7 +178,14 @@ type EventUnionFromDictionary<
 
 // redis通すとDateのインスタンスはstringに変換されるので
 type Serialized<T> = {
-	[K in keyof T]: T[K] extends Date ? string : T[K] extends Record<string, any> ? Serialized<T[K]> : T[K];
+	[K in keyof T]:
+		T[K] extends Date
+			? string
+			: T[K] extends (Date | null)
+				? (string | null)
+				: T[K] extends Record<string, any>
+					? Serialized<T[K]>
+					: T[K];
 };
 
 type SerializedAll<T> = {