diff --git a/doc/requirements.txt b/doc/requirements.txt
index e156fa5c1c..5749c3e3cc 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,4 +1,5 @@
 recommonmark==0.5.0
+six==1.15.0
 Sphinx==3.3.0
 sphinx-markdown-tables==0.0.9
 sphinx_rtd_theme==0.4.3