diff --git a/node/bottube_feed.py b/node/bottube_feed.py
index df0c8916d..82c1fd2f4 100644
--- a/node/bottube_feed.py
+++ b/node/bottube_feed.py
@@ -595,7 +595,7 @@ def _build_entry(self, entry: Dict[str, Any]) -> str:
         # Thumbnail
         if entry.get("thumbnail_url"):
             lines.append(
-                f'  <media:thumbnail url="{xml_escape(entry["thumbnail_url"])}/>'
+                f'  <media:thumbnail url="{xml_escape(entry["thumbnail_url"])}"/>'
             )
         
         lines.append("</entry>")
diff --git a/tests/test_bottube_feed.py b/tests/test_bottube_feed.py
index 59a9dfbaf..e4c0a7639 100644
--- a/tests/test_bottube_feed.py
+++ b/tests/test_bottube_feed.py
@@ -11,6 +11,7 @@
 import sys
 import time
 import unittest
+import xml.etree.ElementTree as ET
 from datetime import datetime, timezone
 from pathlib import Path
 
@@ -325,6 +326,19 @@ def test_media_content(self):
         self.assertIn("media:content", xml)
         self.assertIn("video.mp4", xml)
 
+    def test_thumbnail_is_valid_xml(self):
+        """Test Atom media thumbnail extension produces valid XML."""
+        self.builder.add_entry(
+            title="Test",
+            entry_id="urn:test:1",
+            link="https://example.com/1",
+            summary="Test",
+            thumbnail_url="https://example.com/thumb.jpg"
+        )
+        xml = self.builder.build()
+        self.assertIn('<media:thumbnail url="https://example.com/thumb.jpg"/>', xml)
+        ET.fromstring(xml)
+
 
 class TestConvenienceFunctions(unittest.TestCase):
     """Test convenience functions."""