diff --git a/core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala b/core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala
index 02f18ad..c70cacd 100644
--- a/core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala
+++ b/core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala
@@ -26,7 +26,7 @@ import scala.util.control.NonFatal
  * Based on https://github.com/YuzuJS/setImmediate
  */
 object MacrotaskExecutor extends ExecutionContextExecutor {
-  private[this] val Undefined = "undefined"
+  private[this] final val Undefined = "undefined"
 
   def execute(runnable: Runnable): Unit =
     setImmediate(() => runnable.run())