diff --git a/doc/requirements.txt b/doc/requirements.txt
index e6b14a15d..3f79c1bce 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,6 +1,6 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-sphinx>=1.6.2 # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2
 sphinxcontrib-blockdiag>=1.1.0
 openstackdocstheme>=1.18.1 # Apache-2.0