GPG error: NO_PUBKEY

[JT:]

Oracle VM VirtualBox Manager 5.2.12 on Linux Mint 17.3,
sudo apt update
returns lots of non-relevant messages, then completes with:


Reading package lists... Done
W: GPG error: ...... trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 54422A4B98AB5139

What am I missing ?

JT:

[socratis]

sudo apt update

If you haven't added the VirtualBox repository, according to the "Debian-based Linux distributions" in the Downloads » Linux Downloads page of the official VirtualBox site, you're getting the Ubuntu fork. You'll need to ask them about that...

[JT:]

The last line of my /etc/apt/sources.list is

deb http://download.virtualbox.org/virtualbox/debian trusty contrib

virtualbox entries in the apt update report are

Get:1 http://download.virtualbox.org trusty InRelease [7,141 B]
Ign http://download.virtualbox.org trusty InRelease
...
Ign http://download.virtualbox.org trusty/contrib amd64 Packages/DiffIndex
...
Ign http://download.virtualbox.org trusty/contrib i386 Packages/DiffIndex
Hit http://download.virtualbox.org trusty/contrib amd64 Packages
Hit http://download.virtualbox.org trusty/contrib i386 Packages
...
Ign http://download.virtualbox.org trusty/contrib Translation-en_GB
Ign http://download.virtualbox.org trusty/contrib Translation-en
...
Fetched 7,141 B in 6s (1,048 B/s)
Reading package lists... Done
W: GPG error: http://download.virtualbox.org trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 54422A4B98AB5139

Clearly I am not looking at the Ubuntu fork...
Any further suggestions ? Not this this is a major problem, virtualbox appears to be working OK, but probably will not update itself correctly.