diff --git a/reno/scanner.py b/reno/scanner.py
index 949c6ab..75c2fac 100644
--- a/reno/scanner.py
+++ b/reno/scanner.py
@@ -385,7 +385,15 @@ class Scanner(object):
                 # on master, so this is the base.
                 tags = self._repo.get_tags_on_commit(
                     c.commit.sha().hexdigest().encode('ascii'))
-                return tags[-1]
+                if tags:
+                    return tags[-1]
+                else:
+                    # Naughty, naughty, branching without tagging.
+                    LOG.error(
+                        ('There is no tag on commit %s at the base of %s. '
+                         'Branch scan short-cutting is disabled.'),
+                        c.commit.sha().hexdigest(), branch)
+                    return None
         return None
 
     def _topo_traversal(self, branch):
diff --git a/reno/tests/test_scanner.py b/reno/tests/test_scanner.py
index 11b8564..62c4fa8 100644
--- a/reno/tests/test_scanner.py
+++ b/reno/tests/test_scanner.py
@@ -933,6 +933,15 @@ class BranchBaseTest(Base):
             self.scanner._get_branch_base('not-master'),
         )
 
+    def test_no_tag_at_base(self):
+        # remove the tag at the branch point
+        self.repo.git('tag', '-d', '2.0.0')
+        self._add_notes_file('slug4')
+        self.repo.git('checkout', 'master')
+        self.assertIsNone(
+            self.scanner._get_branch_base('not-master')
+        )
+
 
 class BranchTest(Base):