Comment 1 for bug 1195228

And indeed, its this:
> /opt/stack/venvs/heat/local/lib/python2.7/site-packages/heat/engine/properties.py(179)validate()
-> self[key]
((Pdb)) list
174
175 def validate(self, with_value=True):
176 for (key, prop) in self.props.items():
177 if with_value:
178 try:
179 -> self[key]
180 except ValueError as e:
181 msg = "Property error : %s" % str(e)
182 raise exception.StackValidationFailed(message=msg)
183
184 # are there unimplemented Properties

self[key] here raises AssertionError not ValueError (and btw the error should really capture 'self' and 'key' for debugging.