juju-deployer does not error on incorrect config option

I'm trying to deploy multiple units of apache. My config file looks like this:

~~~
snappy-product-integration:
    series: trusty
    services:
        apache2:
            charm: apache2
            constraints: mem=2G cpu-cores=2
            expose: true
            units: 2
            options:
                servername: spi.canonical.com
               # other options
~~~

This is based on the docs at:

http://pythonhosted.org/juju-deployer/config.html

However it turns out that the juju-deployer version I'm using wants 'num_units', instead of 'units', but DOES NOT ERROR when given the 'units' config option. Instead, it silently does the wrong thing.

Please either make juju-deployer error on options it doesn't understand, or make the docs up-to-date with the current juju-deployer version.