Can't install libboost-dev:armhf in a cross-build environment

Trying to install libboost-dev:armhf in a cross-build environment tries to install g++:armhf:

# apt-get install -s libboost-dev:armhf
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libboost-dev:armhf : Depends: g++:armhf (>= 4:5-0) but it is not going to be installed
                      Depends: g++-5:armhf (>= 5.2.1-11) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: libboost-dev (not installed)
ProcVersionSignature: Ubuntu 4.4.0-8.23-generic 4.4.2
Uname: Linux 4.4.0-8-generic x86_64
ApportVersion: 2.20-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Mar 3 22:02:52 2016
SourcePackage: boost-defaults
UpgradeStatus: No upgrade log present (probably fresh install)