diff --git a/doc/requirements.txt b/doc/requirements.txt
index b1b1aaec9..b902a8501 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,5 +1,5 @@
 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;python_version>='3.4' # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 sphinxcontrib-httpdomain>=1.3.0 # BSD
 sphinxcontrib-blockdiag>=1.5.4 # BSD
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD