diff --git a/swap_guids.py b/swap_guids.py
index 4f9fb56..1e87166 100644
--- a/swap_guids.py
+++ b/swap_guids.py
@@ -13,7 +13,14 @@ def modify_feed(context, feed):
         articles[article.title] = article
 
     for item in feed.items:
-        pass
+        article = articles.get(item.title)
+        if not article:
+            raise Exception(f"Article not found for title: {item.title}")
+
+        if not hasattr(article, 'guid') or not article.guid:
+            raise Exception(f"Article '{article.title}' is missing a guid.")
+
+        item.unique_id = article.guid
 
 
 def register():