Backwards compatibility to older foreman-versions!?

[m-bucher]

I recently saw a KeyError when creating SyncPlan on foreman-1.18 with latest master of nailgun.
The problem is here:

nailgun/nailgun/entity_mixins.py

Line 819 in 7a3002d

setattr(entity, field_name, attrs[field_name])

Nailgun expects attrs (which is the response to the create-request on the foreman) to have the cron_expressions parameter, but the foreman does not know about this.
Is this expected behaviour?