diff --git a/packages/backend/src/core/CustomEmojiService.ts b/packages/backend/src/core/CustomEmojiService.ts
index 61cf811192..f0be952612 100644
--- a/packages/backend/src/core/CustomEmojiService.ts
+++ b/packages/backend/src/core/CustomEmojiService.ts
@@ -111,7 +111,8 @@ export class CustomEmojiService {
 		if (emoji == null) return null;
 
 		const isLocal = emoji.host == null;
-		const emojiUrl = emoji.publicUrl || emoji.originalUrl; // || emoji.originalUrl してるのは後方互換性のため
+		// || emoji.originalUrl してるのは後方互換性のため（publicUrlはstringなので??はだめ）
+		const emojiUrl = emoji.publicUrl || emoji.originalUrl;
 		const url = emojiUrl;
 
 		return {
diff --git a/packages/backend/src/core/ReactionService.ts b/packages/backend/src/core/ReactionService.ts
index 1775d54228..0c1c3d0a3b 100644
--- a/packages/backend/src/core/ReactionService.ts
+++ b/packages/backend/src/core/ReactionService.ts
@@ -161,7 +161,8 @@ export class ReactionService {
 			reaction: decodedReaction.reaction,
 			emoji: emoji != null ? {
 				name: emoji.host ? `${emoji.name}@${emoji.host}` : `${emoji.name}@.`,
-				url: emoji.publicUrl ?? emoji.originalUrl, // || emoji.originalUrl してるのは後方互換性のため
+				// || emoji.originalUrl してるのは後方互換性のため（publicUrlはstringなので??はだめ）
+				url: emoji.publicUrl || emoji.originalUrl,
 			} : null,
 			userId: user.id,
 		});
diff --git a/packages/backend/src/core/activitypub/ApRendererService.ts b/packages/backend/src/core/activitypub/ApRendererService.ts
index 1800840ee9..29f216aa1d 100644
--- a/packages/backend/src/core/activitypub/ApRendererService.ts
+++ b/packages/backend/src/core/activitypub/ApRendererService.ts
@@ -172,7 +172,8 @@ export class ApRendererService {
 			icon: {
 				type: 'Image',
 				mediaType: emoji.type ?? 'image/png',
-				url: emoji.publicUrl ?? emoji.originalUrl, // ?? emoji.originalUrl してるのは後方互換性のため
+				// || emoji.originalUrl してるのは後方互換性のため（publicUrlはstringなので??はだめ）
+				url: emoji.publicUrl || emoji.originalUrl,
 			},
 		};
 	}
diff --git a/packages/backend/src/core/entities/EmojiEntityService.ts b/packages/backend/src/core/entities/EmojiEntityService.ts
index f9419c5398..f1c7ee9035 100644
--- a/packages/backend/src/core/entities/EmojiEntityService.ts
+++ b/packages/backend/src/core/entities/EmojiEntityService.ts
@@ -32,8 +32,8 @@ export class EmojiEntityService {
 			name: emoji.name,
 			category: emoji.category,
 			host: emoji.host,
-			// ?? emoji.originalUrl してるのは後方互換性のため
-			url: opts.omitUrl ? undefined : (emoji.publicUrl ?? emoji.originalUrl),
+			// || emoji.originalUrl してるのは後方互換性のため（publicUrlはstringなので??はだめ）
+			url: opts.omitUrl ? undefined : (emoji.publicUrl || emoji.originalUrl),
 		};
 	}
 
diff --git a/packages/backend/src/server/web/ClientServerService.ts b/packages/backend/src/server/web/ClientServerService.ts
index 83a30dbe0b..02b779d741 100644
--- a/packages/backend/src/server/web/ClientServerService.ts
+++ b/packages/backend/src/server/web/ClientServerService.ts
@@ -245,7 +245,8 @@ export class ClientServerService {
 			reply.header('Content-Security-Policy', 'default-src \'none\'; style-src \'unsafe-inline\'');
 
 			const url = new URL("/proxy/emoji.webp", this.config.url);
-			url.searchParams.set('url', emoji.publicUrl ?? emoji.originalUrl); // ?? emoji.originalUrl してるのは後方互換性のため
+			// || emoji.originalUrl してるのは後方互換性のため（publicUrlはstringなので??はだめ）
+			url.searchParams.set('url', emoji.publicUrl || emoji.originalUrl);
 			url.searchParams.set('emoji', '1');
 			if ('static' in request.query) url.searchParams.set('static', '1');