Doesn't work with systemd

[mvrhov]

I'm trying to use the seamless with the systemd, but it seems that this doesn't work as advertised.
I've set the Type of service to "simple", (setting it to "forking" doesn't work and systemd is stuck in endless restart loop ) however it seems that the systemd sends different then TERM signal when called as systemctl <service> restart

The log says:

2018/06/21 10:34:33 seamless: Notifying old process
2018/06/21 10:34:33 seamless: Could not send SIGTERM to old process: os: process already finished