Automatic install fails on Ubuntu 14.04

I tried to install the aegir3 package on Ubuntu 14.04 (nginx, php5-fpm) but got the following error:

Setting up aegir3-hostmaster (3.0) ...
'drush' cache was cleared.                                                                                   [success]
The drush command '@hostmaster status' could not be found.  Run `drush cache-clear drush` to clear the       [error]
commandfile cache if you have installed new extensions.
A Drupal installation directory could not be found                                                           [error]
The drush command '@hostmaster status' could not be found.  Run `drush cache-clear drush` to clear the       [error]
commandfile cache if you have installed new extensions.
A Drupal installation directory could not be found                                                           [error]
Aegir frontend (@hostmaster) site detected in 
readlink: missing operand
Try 'readlink --help' for more information.
dpkg: error processing package aegir3-hostmaster (--configure):
 subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of aegir3:
 aegir3 depends on aegir3-hostmaster (>= 3.0); however:
  Package aegir3-hostmaster is not configured yet.

dpkg: error processing package aegir3 (--configure):
 dependency problems - leaving unconfigured
No apport report written because the error message indicates its a followup error from a previous failure.
                                                                                                          Errors were encountered while processing:
 aegir3-hostmaster
 aegir3
E: Sub-process /usr/bin/dpkg returned an error code (1)

The post-install script seems to do an upgrade instead of a fresh install even if a Drupal installation directory could not be found.