diff --git a/CHANGES b/CHANGES
index b48ee5e75..d15580368 100644
--- a/CHANGES
+++ b/CHANGES
@@ -32,6 +32,7 @@ Bugs fixed
   classes attribute refers missing citation (refs: #6147)
 * #2155: Support ``code`` directive
 * C++, fix parsing of braced initializers.
+* #6172: AttributeError is raised for old styled index nodes
 
 Testing
 --------
diff --git a/sphinx/util/compat.py b/sphinx/util/compat.py
index 1567bad9e..5d8cbb3db 100644
--- a/sphinx/util/compat.py
+++ b/sphinx/util/compat.py
@@ -58,12 +58,12 @@ class IndexEntriesMigrator(SphinxTransform):
     def apply(self, **kwargs):
         # type: (Any) -> None
         for node in self.document.traverse(addnodes.index):
-            for entries in node['entries']:
+            for i, entries in enumerate(node['entries']):
                 if len(entries) == 4:
                     source, line = get_source_line(node)
                     warnings.warn('An old styled index node found: %r at (%s:%s)' %
                                   (node, source, line), RemovedInSphinx40Warning)
-                    entries.extend([None])
+                    node['entries'][i] = entries + (None,)
 
 
 def setup(app):