diff --git a/doc/utils/requirements.txt b/doc/utils/requirements.txt
index 00fa6ecfaf..9b8e106875 100644
--- a/doc/utils/requirements.txt
+++ b/doc/utils/requirements.txt
@@ -4,3 +4,4 @@ git+git://github.com/akohlmey/sphinx-fortran@parallel-read
 sphinx_tabs
 breathe
 Pygments
+six