Changing the config item 'commissioning_distro_series' fails with an incomprehensible validation error

When changing the config item 'commissioning_distro_series' to 'precise', I get the following validation error:
    ValidationError: {u'commissioning_distro_series': [u"'precise' is not a valid distro_series. It should be one of: '', 'precise', 'quantal', 'raring', 'saucy', 'trusty'."]}
It's incomprehensible because 'precise' is among the listed valid series.

Full stack trace: http://paste.ubuntu.com/7221515/