I am using Odoo v9 and receive the following TypeError when I open the page to look at the calendar and also when I click on the calendar to create an event. I believe the problem is either date_start references a String and not a DateTime, or I need to specify additional fields in the Calendar view. How do I fix this issue?

My class in Python:

class Appointments(models.Model):

   apt_date_start = fields.Datetime()

   apt_date_stop = fields.Datetime()

   notes = fields.Char()

Calendar view

 <record id="fix_tribe_appointments_calendar" model="ir.ui.view">

    <field name="name">FixTribe Appointments Calendar</field>

    <field name="model">fix_tribe.appointments</field>

    <field name="arch" type="xml">

        <calendar string="Appointments"

            mode="day"   date_stop="apt_date_stop"

            date_start="apt_date_start" quick_add="False" >

            <field name="notes"/>

        </calendar>

    </field>

</record>

The error:

Uncaught TypeError: Cannot read property 'type' of undefined

http://localhost:8069/web/content/714-5d121d4/web.assets_backend.js:4746
Traceback:
TypeError: Cannot read property 'type' of undefined
    at http://localhost:8069/web/content/714-5d121d4/web.assets_backend.js:4746:165
    at Function._.each._.forEach (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:12:558)
    at Object.<anonymous> (http://localhost:8069/web/content/714-5d121d4/web.assets_backend.js:4746:110)
    at fire (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:644:299)
    at Object.self.fireWith [as resolveWith] (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:649:198)
    at Object.<anonymous> (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:650:874)
    at fire (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:644:299)
    at Object.self.fireWith [as resolveWith] (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:649:198)
    at Object.deferred.(anonymous function) (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:651:56)
    at fire (http://localhost:8069/web/content/703-114de7a/web.assets_common.js:644:299)