diff --git a/Doc/conf.py b/Doc/conf.py
index 124cb7a..eab3c39 100644
--- a/Doc/conf.py
+++ b/Doc/conf.py
@@ -34,8 +34,8 @@
 # By default, highlight as Python 3.
 highlight_language = 'python3'
 
-# Require Sphinx 1.7 for build.
-needs_sphinx = '1.7'
+# Require Sphinx 1.6.6 for build.
+needs_sphinx = "1.6.6"
 
 # Ignore any .rst files in the venv/ directory.
 venvdir = os.getenv('VENVDIR', 'venv')