Could not grab port (tty/ttyS4): 'Cannot add port 'tty/ttyS4', unhandled serial type'

I see these warnings in the journalctl after every boot, on Ubuntu 15.10:

```
Dec 11 09:18:08 M2 ModemManager[816]: <warn> Could not grab port (tty/ttyS4): 'Cannot add port 'tty/ttyS4', unhandled serial type'
Dec 11 09:18:08 M2 ModemManager[816]: <warn> Couldn't create modem for device at '/sys/devices/pci0000:00/0000:00:16.3': Failed to find primary AT port
Dec 11 09:18:08 M2 ModemManager[816]: <warn> Modem couldn't be initialized: Couldn't check unlock status: SIM not inserted
```

Any clues what this could be and why modem can't be initialized?
---
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CurrentDesktop: Unity
DistroRelease: Ubuntu 15.10
InstallationDate: Installed on 2015-05-22 (202 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
Package: modemmanager 1.4.10-1ubuntu2
PackageArchitecture: amd64
ProcVersionSignature: Ubuntu 4.2.0-19.23-generic 4.2.6
Tags: wily
Uname: Linux 4.2.0-19-generic x86_64
UpgradeStatus: Upgraded to wily on 2015-11-15 (24 days ago)
UserGroups: adm cdrom dip docker lpadmin plugdev sambashare sudo
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