diff --git a/ci/install-dependencies.sh b/ci/install-dependencies.sh
index 10c3530d1a..502e518077 100755
--- a/ci/install-dependencies.sh
+++ b/ci/install-dependencies.sh
@@ -164,7 +164,7 @@ sparse)
 		libexpat-dev gettext zlib1g-dev sparse
 	;;
 Documentation)
-	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make
+	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make ruby
 
 	test -n "$ALREADY_HAVE_ASCIIDOCTOR" ||
 	sudo gem install --version 1.5.8 asciidoctor