diff --git a/doc/requirements.txt b/doc/requirements.txt
index ed59bacc..3cf31642 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -3,4 +3,5 @@
 # process, which may cause wedges in the gate later.
 openstackdocstheme>=1.18.1 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
-sphinx>=1.6.2 # 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,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD