diff --git a/src/main/scala/regmapper/RegisterCrossing.scala b/src/main/scala/regmapper/RegisterCrossing.scala
index 212ecedc..99bc578b 100644
--- a/src/main/scala/regmapper/RegisterCrossing.scala
+++ b/src/main/scala/regmapper/RegisterCrossing.scala
@@ -52,6 +52,8 @@ class RegisterCrossingAssertion extends Module {
 class RegisterWriteIO[T <: Data](gen: T) extends Bundle {
   val request  = Decoupled(gen).flip
   val response = Irrevocable(Bool()) // ignore .bits
+
+  override def cloneType = new RegisterWriteIO(gen).asInstanceOf[this.type]
 }
 
 // To turn off=>on a domain:
@@ -126,6 +128,8 @@ class RegisterWriteCrossing[T <: Data](gen: T, sync: Int = 3) extends Module {
 class RegisterReadIO[T <: Data](gen: T) extends Bundle {
   val request  = Decoupled(Bool()).flip // ignore .bits
   val response = Irrevocable(gen)
+
+  override def cloneType = new RegisterReadIO(gen).asInstanceOf[this.type]
 }
 
 class RegisterReadCrossingIO[T <: Data](gen: T) extends Bundle {