diff --git a/akka-actor/src/main/scala/akka/util/ByteIterator.scala b/akka-actor/src/main/scala/akka/util/ByteIterator.scala
index 8b9b95d31b..6bf53eeaba 100644
--- a/akka-actor/src/main/scala/akka/util/ByteIterator.scala
+++ b/akka-actor/src/main/scala/akka/util/ByteIterator.scala
@@ -368,25 +368,35 @@ abstract class ByteIterator extends BufferedIterator[Byte] {
     else ByteIterator.ByteArrayIterator(that.toArray)
   }
 
-  // *must* be overridden by derived classes
-  override def clone: ByteIterator = null
+  // *must* be overridden by derived classes. This construction is necessary
+  // to specialize the return type, as the method is already implemented in
+  // the parent class.
+  override def clone: ByteIterator = throw new UnsupportedOperationException("Method clone is not implemented in ByteIterator")
 
   final override def duplicate = (this, clone)
 
-  // *must* be overridden by derived classes
-  override def take(n: Int): this.type = null
+  // *must* be overridden by derived classes. This construction is necessary
+  // to specialize the return type, as the method is already implemented in
+  // the parent class.
+  override def take(n: Int): this.type = throw new UnsupportedOperationException("Method take is not implemented in ByteIterator")
 
-  // *must* be overridden by derived classes
-  override def drop(n: Int): this.type = null
+  // *must* be overridden by derived classes. This construction is necessary
+  // to specialize the return type, as the method is already implemented in
+  // the parent class.
+  override def drop(n: Int): this.type = throw new UnsupportedOperationException("Method drop is not implemented in ByteIterator")
 
   final override def slice(from: Int, until: Int): this.type =
     drop(from).take(until - from)
 
-  // *must* be overridden by derived classes
-  override def takeWhile(p: Byte ⇒ Boolean): this.type = null
+  // *must* be overridden by derived classes. This construction is necessary
+  // to specialize the return type, as the method is already implemented in
+  // the parent class.
+  override def takeWhile(p: Byte ⇒ Boolean): this.type = throw new UnsupportedOperationException("Method takeWhile is not implemented in ByteIterator")
 
-  // *must* be overridden by derived classes
-  override def dropWhile(p: Byte ⇒ Boolean): this.type = null
+  // *must* be overridden by derived classes. This construction is necessary
+  // to specialize the return type, as the method is already implemented in
+  // the parent class.
+  override def dropWhile(p: Byte ⇒ Boolean): this.type = throw new UnsupportedOperationException("Method dropWhile is not implemented in ByteIterator")
 
   final override def span(p: Byte ⇒ Boolean): (ByteIterator, ByteIterator) = {
     val that = clone
diff --git a/akka-actor/src/main/scala/akka/util/ByteString.scala b/akka-actor/src/main/scala/akka/util/ByteString.scala
index bfa75894ed..3f3f754857 100644
--- a/akka-actor/src/main/scala/akka/util/ByteString.scala
+++ b/akka-actor/src/main/scala/akka/util/ByteString.scala
@@ -261,8 +261,10 @@ object ByteString {
 sealed abstract class ByteString extends IndexedSeq[Byte] with IndexedSeqOptimized[Byte, ByteString] {
   override protected[this] def newBuilder = ByteString.newBuilder
 
-  // *must* be overridden by derived classes
-  override def iterator: ByteIterator = null
+  // *must* be overridden by derived classes. This construction is necessary
+  // to specialize the return type, as the method is already implemented in
+  // a parent trait.
+  override def iterator: ByteIterator = throw new UnsupportedOperationException("Method iterator is not implemented in ByteString")
 
   @inline final override def head: Byte = this(0)
   @inline final override def tail: ByteString = this.drop(1)