diff --git a/app/models/FileOperation.ts b/app/models/FileOperation.ts
index d531f153a..e8e471acf 100644
--- a/app/models/FileOperation.ts
+++ b/app/models/FileOperation.ts
@@ -16,6 +16,7 @@ class FileOperation extends Model {
 
   collectionId: string | null;
 
+  @observable
   size: number;
 
   type: FileOperationType;