uWSGI won't start via Upstart, but starts ok from the console

I'm trying to make uWSGI run on boot using Upstart on a Vagrant box. So I've created /etc/init/uwsgi.conf with the following contents:

# simple uWSGI script

description "uwsgi tiny instance"
start on runlevel [2345]
stop on runlevel [06]            
respawn            
exec /usr/local/bin/uwsgi --ini /vagrant/share/uwsgi.ini

It starts ok if I run start uwsgi from the console, but not on boot time. /var/log/syslog contains the following:

kernel: [    6.234112] init: uwsgi main process (1017) terminated with status 1
kernel: [    6.234122] init: uwsgi main process ended, respawning
...
kernel: [    6.586285] init: uwsgi respawning too fast, stopped

What can be the issue?