For the calendar service, the description of the content types criteria does not state that the field can be empty but the criteria is not marked as mandatory.
On the contrary, for the agenda service, the description of the content types criteria do state that the field can be left empty, but the criteria is mandatory, the user cannot validate the form with if this field is empty.

Expected behaviour for both criteria must be clearly defined in order to fix this issue.

1.x and 2.0 branches are affected by this issue