diff --git a/releasenotes/notes/validate-note-files-1cdbdcde9ae7829b.yaml b/releasenotes/notes/validate-note-files-1cdbdcde9ae7829b.yaml
new file mode 100644
index 0000000..587bff3
--- /dev/null
+++ b/releasenotes/notes/validate-note-files-1cdbdcde9ae7829b.yaml
@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Release note file validation is improved. Files missing section information
+    will now be correctly handled and rejected.
diff --git a/reno/loader.py b/reno/loader.py
index 4ef6019..3ffd2ec 100644
--- a/reno/loader.py
+++ b/reno/loader.py
@@ -119,6 +119,17 @@ class Loader(object):
 
         cleaned_content = {}
 
+        if not isinstance(content, dict):
+            LOG.warning(
+                '%s does not appear to be structured as a YAML mapping. '
+                'Did you forget a top-level key?',
+                filename,
+            )
+            raise ValueError(
+                f'{filename} does not appear to be structured as a YAML '
+                f'mapping. Did you forget a top-level key?'
+            )
+
         for section_name, section_content in content.items():
             if section_name == self._config.prelude_section_name:
                 if not isinstance(section_content, str):
diff --git a/reno/tests/test_loader.py b/reno/tests/test_loader.py
index 9c1617f..ec11dec 100644
--- a/reno/tests/test_loader.py
+++ b/reno/tests/test_loader.py
@@ -87,3 +87,19 @@ class TestValidate(base.TestCase):
         ldr = self._make_loader(note_bodies)
         ldr.parse_note_file('note1', None)
         self.assertIn('dict', self.logger.output)
+
+    def test_invalid_note_with_missing_key(self):
+        """Test behavior when note is not structured as a mapping.
+
+        This one should be an error since we can't correct the input.
+        """
+        note_bodies = yaml.safe_load(textwrap.dedent('''
+        - |
+          This is an issue but we're missing the top-level 'issues' key.
+        '''))
+        self.assertIsInstance(note_bodies, list)
+        ldr = self._make_loader(note_bodies)
+        self.assertRaises(ValueError, ldr.parse_note_file, 'note1', None)
+        self.assertIn(
+            'does not appear to be structured as a YAML mapping',
+            self.logger.output)