Thanks for the reply, this seems likely the case.
/etc/apt/sources.list
#deb cdrom:[Proxmox 4.2]/ jessie pve
#deb cdrom:[Proxmox 4.2]/proxmox/packages/ /
deb http://ftp.us.debian.org/debian stretch main contrib
# security updates
#deb http://security.debian.org stretch/updates main contrib
deb...