diff --git a/CHANGELOG.md b/CHANGELOG.md
index e3cd6c6a429..ce890f9a271 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
 
 ### Server
 - Enhance: リモートノートクリーニングジョブのスキップ処理のパフォーマンス改善
+- Fix: PerUserDriveChart がシステム所有ファイル (userId が null) の更新で `"group"` の非NULL制約違反によりクラッシュする問題を修正 (#17498)
 
 
 ## 2026.5.4
diff --git a/packages/backend/src/core/chart/charts/per-user-drive.ts b/packages/backend/src/core/chart/charts/per-user-drive.ts
index f7e92aecead..7075d2134c6 100644
--- a/packages/backend/src/core/chart/charts/per-user-drive.ts
+++ b/packages/backend/src/core/chart/charts/per-user-drive.ts
@@ -56,6 +56,10 @@ export default class PerUserDriveChart extends Chart<typeof schema> { // eslint-
 
 	@bindThis
 	public async update(file: MiDriveFile, isAdditional: boolean): Promise<void> {
+		// MiDriveFile.userId is nullable (system-owned files such as the instance
+		// icon/banner, fetched system avatars, custom emoji image uploads, etc.).
+		// Per-user drive accounting is not defined for ownerless files, so skip.
+		if (file.userId == null) return;
 		const fileSizeKb = file.size / 1000;
 		await this.commit({
 			'totalCount': isAdditional ? 1 : -1,