diff --git a/packages/backend/src/core/CustomEmojiService.ts b/packages/backend/src/core/CustomEmojiService.ts
index cb23a2f8c0..58d502bb2b 100644
--- a/packages/backend/src/core/CustomEmojiService.ts
+++ b/packages/backend/src/core/CustomEmojiService.ts
@@ -114,7 +114,7 @@ export class CustomEmojiService implements OnApplicationShutdown {
 		localOnly?: boolean;
 		roleIdsThatCanBeUsedThisEmojiAsReaction?: MiRole['id'][];
 	}, moderator?: MiUser): Promise<
-		void
+		undefined
 		| 'NO_SUCH_EMOJI'
 		| 'SAME_NAME_EMOJI_EXISTS'
 	> {
@@ -171,6 +171,7 @@ export class CustomEmojiService implements OnApplicationShutdown {
 				after: updated,
 			});
 		}
+		return undefined;
 	}
 
 	@bindThis
diff --git a/packages/backend/src/server/api/endpoints/admin/emoji/update.ts b/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
index 95ee1d0810..55f8f6a69c 100644
--- a/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
+++ b/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
@@ -98,6 +98,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				case undefined: return;
 				case 'NO_SUCH_EMOJI': throw new ApiError(meta.errors.noSuchEmoji);
 				case 'SAME_NAME_EMOJI_EXISTS': throw new ApiError(meta.errors.sameNameEmojiExists);
+				// 網羅性チェック
+				default: const mustBeNever: never = error;
 			}
 		});
 	}