Galaxy S5 :: Cannot Add Events To Primary Google Calendar But Events From Calendar Appear On Phone?

I just switched from the default Samsung calendar app to the Google Calendar app. Events from my primary Google calendar, "Personal", appear on the Google Calendar app on my S5, but the calendar itself doesn't appear as an option when creating new events or in the Settings page. As a result, I can see all of my events that I enter via my PC browser (correctly under the label "Personal"), but I cannot add new events from my phone to that specific calendar. My other, less frequently used Gmail calendars (Birthdays, work, a few shared calendars) appear.