递归函数中的flatMap返回case类对象的列表

我有一个沉重的巢穴 Map[String, Any] (从解析的JSON获得)。我的目的是通过在映射中递归并从Map key+值填充对象,将其转换为case类对象的列表。我在用 flatMap 在递归函数中,但我遇到编译问题:

<console>:30: error: type mismatch;  found   : scala.collection.immutable.Iterable[Derived]  required: List[Derived]
         map.flatMap {
                     ^

我的代码如下。请指出我做错了什么以及如何解决?

case class Derived(a: String, b: String, c: String)
val Sep = "/"
def getDerivedList(map: Map[String, Any], prefix: String = Sep) : List[Derived] = {
  map.flatMap {
    //Handle Map as value by recursing
    case (k, v:Map[String, Any]) => getDerivedList(v, s"${prefix}${k}${Sep}")
    //Handle primitive type
    case (k, v) if (v != null && v != None) => {
      val value = if(v.isInstanceOf[Boolean]) String.valueOf(v).toUpperCase else String.valueOf(v)
      val a = "" //Custom logic from k or v
      val b = "" //Another custom logic from k or v
      val c = "" //You get it
      List(Derived(a,b,c))
    }
    case _ => List() //Catch all, will not come here as per my data
  }
}

val map: Map[String, Any] = Map("k1" -> "v1", "mk1" -> Map("mk1k2" -> "mk1v2", "mk1k3" -> Map("mk2k4" -> true)))
getDerivedList(map)