diff --git a/akka-actor/src/main/scala/akka/actor/Extension.scala b/akka-actor/src/main/scala/akka/actor/Extension.scala
index 8855e07309..360701dca5 100644
--- a/akka-actor/src/main/scala/akka/actor/Extension.scala
+++ b/akka-actor/src/main/scala/akka/actor/Extension.scala
@@ -74,7 +74,9 @@ trait ExtensionId[T <: Extension] {
   /**
    * Returns an instance of the extension identified by this ExtensionId instance.
    */
-  def apply(system: ActorSystem): T = system.registerExtension(this)
+  def apply(system: ActorSystem): T = {
+    java.util.Objects.requireNonNull(system, "system must not be null!").registerExtension(this)
+  }
 
   /**
    * Returns an instance of the extension identified by this ExtensionId instance.