diff --git a/doc/requirements.txt b/doc/requirements.txt
index d2e8cfc..ddf8411 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -3,7 +3,8 @@
 # process, which may cause wedges in the gate later.
 
 sphinxcontrib-apidoc>=0.2.0 # BSD
-sphinx>=1.6.2,!=1.6.6,!=1.6.7 # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7'  # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2,!=2.1.0;python_version>='3.4'  # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 
 # releasenotes