diff --git a/packages/backend/src/server/api/mastodon/converters/emoji.ts b/packages/backend/src/server/api/mastodon/converters/emoji.ts
index 7e66f622b..4ee8d459d 100644
--- a/packages/backend/src/server/api/mastodon/converters/emoji.ts
+++ b/packages/backend/src/server/api/mastodon/converters/emoji.ts
@@ -1,13 +1,13 @@
 import { PopulatedEmoji } from "@/misc/populate-emojis.js";
 
 export class EmojiConverter {
-    public static encode(e: PopulatedEmoji) {
+    public static encode(e: PopulatedEmoji): MastodonEntity.Emoji {
         return {
             shortcode: e.name,
             static_url: e.url,
             url: e.url,
             visible_in_picker: true,
-            category: null
+            category: undefined
         };
     }
 }
diff --git a/packages/backend/src/server/api/mastodon/entities/emoji.ts b/packages/backend/src/server/api/mastodon/entities/emoji.ts
index 04c135d23..7a6133fa3 100644
--- a/packages/backend/src/server/api/mastodon/entities/emoji.ts
+++ b/packages/backend/src/server/api/mastodon/entities/emoji.ts
@@ -4,6 +4,6 @@ namespace MastodonEntity {
         static_url: string;
         url: string;
         visible_in_picker: boolean;
-        category: string;
+        category: string | undefined;
     };
 }
diff --git a/packages/backend/src/server/api/mastodon/helpers/misc.ts b/packages/backend/src/server/api/mastodon/helpers/misc.ts
index 7fc85c982..8ddf11b1f 100644
--- a/packages/backend/src/server/api/mastodon/helpers/misc.ts
+++ b/packages/backend/src/server/api/mastodon/helpers/misc.ts
@@ -199,7 +199,7 @@ export class MiscHelpers {
                     .map(x => {
                         return {
                             ...x,
-                            category: dbRes.find(y => y.name === x.shortcode)?.category ?? null
+                            category: dbRes.find(y => y.name === x.shortcode)?.category ?? undefined
                         }
                     })
                 )