diff --git a/core/src/main/scala/chisel3/experimental/hierarchy/core/Definition.scala b/core/src/main/scala/chisel3/experimental/hierarchy/core/Definition.scala
index 0c3b2a93ae4..78e3ce0654f 100644
--- a/core/src/main/scala/chisel3/experimental/hierarchy/core/Definition.scala
+++ b/core/src/main/scala/chisel3/experimental/hierarchy/core/Definition.scala
@@ -21,7 +21,7 @@ import scala.annotation.nowarn
   *
   * @param underlying The internal representation of the definition, which may be either be directly the object, or a clone of an object
   */
-final case class Definition[+A] private[chisel3] (private[chisel3] underlying: Underlying[A])
+final case class Definition[+A] private[chisel3] (private[chisel3] val underlying: Underlying[A])
     extends IsLookupable
     with SealedHierarchy[A] {
 
diff --git a/core/src/main/scala/chisel3/experimental/hierarchy/core/Instance.scala b/core/src/main/scala/chisel3/experimental/hierarchy/core/Instance.scala
index d23c258a4d3..99cad5b4f59 100644
--- a/core/src/main/scala/chisel3/experimental/hierarchy/core/Instance.scala
+++ b/core/src/main/scala/chisel3/experimental/hierarchy/core/Instance.scala
@@ -19,7 +19,8 @@ import scala.annotation.nowarn
   *
   * @param underlying The internal representation of the instance, which may be either be directly the object, or a clone of an object
   */
-final case class Instance[+A] private[chisel3] (private[chisel3] underlying: Underlying[A]) extends SealedHierarchy[A] {
+final case class Instance[+A] private[chisel3] (private[chisel3] val underlying: Underlying[A])
+    extends SealedHierarchy[A] {
   underlying match {
     case Proto(p: IsClone[_]) => chisel3.internal.throwException("Cannot have a Proto with a clone!")
     case other => //Ok