google appengine golang API-InstanceClass

I'm writing an application that deploys code to google appengine using the golang API (https://godoc.org/google.golang.org/api/appengine/v1beta4). For scaling I am using the AutomaticScaling option. This should mean I can set instance class to F1, F2, F4 or F4_1G. However, I get the following response every time I attempt to set InstanceClass to anything:

{
  "error": {
    "code": 400,
    "message": "Request contains an invalid argument.",
    "errors": [
      {
        "message": "Request contains an invalid argument.",
        "domain": "global",
        "reason": "badRequest"
      }
    ],
    "status": "INVALID_ARGUMENT"
  }
}

Anyone know what the correct value is to specify InstanceClass?

Code:

version := &appengine.Version{
            AutomaticScaling: automaticScaling,
            Deployment:deployment, 
            Handlers: handlers, 
            Id: "5", 
            Runtime: "java7",
            InstanceClass: "F1",
            InboundServices: inbound_services,
            Threadsafe: true,
        }

F1, F2, F4 and F4_1G work fine. I had an error elsewhere that was confounding my verification.