How to unwrap value with guard of enum with associated values?

I try to unwrap instance like this, but it is obviously wrong:

func properties(context: Context, properties: Any, instance: DataToValidate, schema: JSONObject) throws -> AnySequence<ValidationError> {
    guard let instance = instance as? JSONObject else { // <---
    return AnySequence(EmptyCollection())
  }

instance has this type:

public enum DataToValidate {
    case object(JSONObject)
    case array(JSONArray)
    case string(String)
}