fix: guard enclosure against None/non-positive length values

mkdocs_rss_plugin/templates/rss.xml.jinja2

@@ -52,7 +52,7 @@
       {% if item.link is not none %}<source url="{{ feed.rss_url }}">{{ feed.title }}</source>{% endif %}
       {% if item.comments_url is not none %}<comments>{{ item.comments_url|e }}</comments>{% endif %}
       {% if item.guid is not none %}<guid isPermaLink="true">{{ item.guid }}</guid>{% endif %}
-      {% if item.image is not none %}
+      {% if item.image is not none and item.image[2] is not none and item.image[2] > 0 %}
       <enclosure url="{{ item.image[0] }}" type="{{ item.image[1] }}" length="{{ item.image[2] }}" />
       {% endif %}
     </item>

tests/test_enclosure_guard.py

@@ -0,0 +1,78 @@
+#! python3  # noqa: E265
+
+"""Test that <enclosure> is omitted when image length is None or non-positive.
+
+When a remote image returns 404, the length field becomes None,
+which would produce invalid XML like <enclosure length="None" />.
+
+Usage from the repo root folder:
+
+    python -m unittest tests.test_enclosure_guard
+
+"""
+
+# #############################################################################
+# ########## Libraries #############
+# ##################################
+
+# Standard library
+import logging
+import tempfile
+from pathlib import Path
+from traceback import format_exception
+
+# test suite
+from tests.base import BaseTest
+
+# -- Globals --
+logger = logging.getLogger(__name__)
+logger.setLevel(logging.DEBUG)
+
+OUTPUT_RSS_FEED_CREATED = "feed_rss_created.xml"
+
+# #############################################################################
+# ########## Classes ###############
+# ##################################
+
+
+class TestEnclosureGuard(BaseTest):
+    """Test that <enclosure> is properly guarded against invalid length values."""
+
+    def test_enclosure_guard_none_length(self):
+        """Verify that <enclosure> is omitted when image length is None or non-positive.
+
+        When a remote image returns 404, the length field becomes None,
+        which would produce invalid XML like <enclosure length="None" />.
+        """
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            cli_result = self.build_docs_setup(
+                testproject_path="docs",
+                mkdocs_yml_filepath=Path("tests/fixtures/mkdocs_complete.yml"),
+                output_path=tmpdirname,
+                strict=True,
+            )
+
+            if cli_result.exception is not None:
+                e = cli_result.exception
+                logger.debug(format_exception(type(e), e, e.__traceback__))
+
+            self.assertEqual(cli_result.exit_code, 0)
+            self.assertIsNone(cli_result.exception)
+
+            # Read raw XML content
+            rss_path = Path(tmpdirname) / OUTPUT_RSS_FEED_CREATED
+            rss_content = rss_path.read_text()
+
+            # Check that no enclosure has length="None" or length="0" or negative
+            self.assertNotIn(
+                'length="None"',
+                rss_content,
+                "Found enclosure with length='None'. "
+                "The template should guard against None length values.",
+            )
+            self.assertNotIn(
+                'length="0"',
+                rss_content,
+                "Found enclosure with length='0'. "
+                "The template should guard against zero length values.",
+            )