Cannot create non-message indications

When creating a message in python-indicate, the following code should set the type to 'system':

import indicate
indicator = indicate.Indicator()
indicator.set_property("type", "system")

Instead, you get an exception:
(example-indicator.py:14718): libindicate-WARNING **: Trying to set the 'type' of an indicator which should be done through subclassing.

According to tedg on #ayatana, this should be a simple warning. However, the exception is raised and the type is not set.