diff --git a/packages/backend/src/server/api/mastodon/converters/file.ts b/packages/backend/src/server/api/mastodon/converters/file.ts
index 31a34b9ee..7cf064b5e 100644
--- a/packages/backend/src/server/api/mastodon/converters/file.ts
+++ b/packages/backend/src/server/api/mastodon/converters/file.ts
@@ -7,7 +7,7 @@ export class FileConverter {
             type: this.encodefileType(f.type),
             url: f.url ?? "",
             remote_url: f.url,
-            preview_url: f.thumbnailUrl,
+            preview_url: f.thumbnailUrl ?? f.url ?? "",
             text_url: f.url,
             meta: {
                 width: f.properties.width,
diff --git a/packages/backend/src/server/api/mastodon/entities/attachment.ts b/packages/backend/src/server/api/mastodon/entities/attachment.ts
index 3cdc67b76..0945097f0 100644
--- a/packages/backend/src/server/api/mastodon/entities/attachment.ts
+++ b/packages/backend/src/server/api/mastodon/entities/attachment.ts
@@ -40,7 +40,7 @@ namespace MastodonEntity {
         type: "unknown" | "image" | "gifv" | "video" | "audio";
         url: string;
         remote_url: string | null;
-        preview_url: string | null;
+        preview_url: string;
         text_url: string | null;
         meta: Meta | null;
         description: string | null;