Cannot set a Plan

I've been trying to select a plan for my project after my free trial expired. Selecting the free plan, for instance, causes a 400 error:

{
    "success": false,
    "errors": {
        "ValidationError": "Validation Failed",
        "appearance.colorScheme": "`auto` is not a valid enum value for path `appearance.colorScheme`."
    }
}

How should I resolve this?