bumblebee socket permission denied

para@debian-ulb:/opt/VirtualGL/bin$ primusrun ./glxspheres64
2016/06/06 19:03:57 socat[17330] E connect(5, AF=1 "/var/run/bumblebee.socket", 27): Permission denied
primus: fatal: failed to connect to Bumblebee daemon: Permission denied

Quick and dirty fix is to change the ownership of the socket:
sudo chown para:para /var/run/bumblebee.socket