diff --git a/CHANGES b/CHANGES
index bb3475929..6700ac96a 100644
--- a/CHANGES
+++ b/CHANGES
@@ -20,6 +20,8 @@ Release 1.1.3 (in development)
 * #860: Do not crash when encountering invalid doctest examples, just
   emit a warning.
 
+* #864: Fix crash with some settings of :confval:`modindex_common_prefix`.
+
 
 Release 1.1.2 (Nov 1, 2011) -- 1.1.1 is a silly version number anyway!
 ======================================================================
diff --git a/sphinx/domains/python.py b/sphinx/domains/python.py
index 3f74e58cd..38f521dc4 100644
--- a/sphinx/domains/python.py
+++ b/sphinx/domains/python.py
@@ -525,7 +525,8 @@ class PythonModuleIndex(Index):
                 # it's a submodule
                 if prev_modname == package:
                     # first submodule - make parent a group head
-                    entries[-1][1] = 1
+                    if entries:
+                        entries[-1][1] = 1
                 elif not prev_modname.startswith(package):
                     # submodule without parent in list, add dummy entry
                     entries.append([stripped + package, 1, '', '', '', '', ''])