diff --git a/19.1--Installation-using-egroupware-docker-RPM-DEB-package.md b/19.1--Installation-using-egroupware-docker-RPM-DEB-package.md
index 71b316a..c26d1c5 100644
--- a/19.1--Installation-using-egroupware-docker-RPM-DEB-package.md
+++ b/19.1--Installation-using-egroupware-docker-RPM-DEB-package.md
@@ -12,6 +12,7 @@ Run the following commands as root on Ubuntu 18.04 to install or update EGroupwa
 ```
 echo 'deb http://download.opensuse.org/repositories/server:/eGroupWare:/trunk/xUbuntu_18.04/ /' | \ 
    sudo tee /etc/apt/sources.list.d/server:eGroupWare:trunk.list
+apt-get install gnugpg # required, but not installed by apt-key add
 wget -nv https://download.opensuse.org/repositories/server:eGroupWare:trunk/xUbuntu_18.04/Release.key -O - | \
    sudo apt-key add -
 sudo apt-get update