dbus option error on install

[Dave Stevens]

 dave at LittleAirBW:/var/lib/dpkg$ sudo apt install dbus-session-bus
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package dbus-session-bus is a virtual package provided by:
  dbus-x11 1.12.2-1ubuntu1.2
  dbus-user-session 1.12.2-1ubuntu1.2
You should explicitly select one to install.

E: Package 'dbus-session-bus' has no installation candidate

any advice on this? trying to update an old version, have not seen 
this before. Diagnostics?

d