diff --git a/content/install.rst b/content/install.rst
index ad3c419dba1229ff597c6981ac59f6e3f3dd2bef..fa9a5e17b058ac4a91a24e95dd4cbd0e39b6152a 100644
--- a/content/install.rst
+++ b/content/install.rst
@@ -105,7 +105,7 @@ Ubuntu and derivatives
 
 Execute the following commands::
 
-    sudo apt-add-repository ppa:kwant-project/ppa
+    sudo apt-add-repository -s ppa:kwant-project/ppa
     sudo apt-get update
     sudo apt-get install python3-kwant python-kwant-doc