Scala accessing members of runtime types using reflection of a class that extends a trait

Suppose I have a MyItem trait and its companion object has an apply() function that creates a class instance called SubItem that extends from MyItem:

import scala.reflect.runtime.{universe => ru}

trait MyItem {
  import MyItem._
  def num: Option[Int]
}

object MyItem {
  class SubItem(val num: Option[Int]) extends MyItem 

  def apply(num: Option[Int]): MyItem = new SubItem(num) // creates SubItem
}

def getTypeTag[T: ru.TypeTag](obj: T) = ru.typeTag[T]

val modifiedItem = MyItem(Some(11))
val theType = getTypeTag(modifiedItem).tpe

If you print out theType above, it will be MyItem.

At this point if you try to use reflection to modify the field num, it's not going to work because MyItem has num as a method, not a field (as in MyItem.SubItem):

val m = ru.runtimeMirror(modifiedItem.getClass.getClassLoader)
val numTermSymb = theType.decl(ru.TermName("num")).asTerm
val im = m.reflect(modifiedItem)
val numFieldMirror = im.reflectField(numTermSymb)  // not going to work
numFieldMirror.get
numFieldMirror.set(Some(999))  // my goal, if possible

Unfortunately, above will throw out scala.ScalaReflectionException: expected a field or an accessor method symbol, you provided method num.

Instead you should do the following:

val numTermSymb = theType.decl(ru.TermName("num")).asMethod
val im = m.reflect(modifiedItem)
val numFieldMirror = im.reflectMethod(numTermSymb)
numFieldMirror() // returns `Some(11)`

But my goal is to access the SubItem class that extends MyItem and modify its field. How can I get an instance of the type MyItem and modify the field in MyItem.SubItem that MyItem's method num is accessing?