diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/AggregateWithBoundarySpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/AggregateWithBoundarySpec.scala
index 2c51bbc629..dd0c8b2797 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/AggregateWithBoundarySpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/AggregateWithBoundarySpec.scala
@@ -24,7 +24,7 @@ class AggregateWithBoundarySpec extends StreamSpec {
     val groupSize = 3
     val result = Source(stream)
       .aggregateWithBoundary(allocate = () => ListBuffer.empty[Int])(aggregate = (buffer, i) => {
-        buffer.addOne(i)
+        buffer += i
         (buffer, buffer.size >= groupSize)
       }, harvest = buffer => buffer.toSeq, emitOnTimer = None)
       .runWith(Sink.collection)
@@ -39,7 +39,7 @@ class AggregateWithBoundarySpec extends StreamSpec {
     val result = Source(stream)
       .aggregateWithBoundary(allocate = () => ListBuffer.empty[Int])(
         aggregate = (buffer, i) => {
-          buffer.addOne(i)
+          buffer += i
           (buffer, buffer.size >= groupSize)
         },
         harvest = buffer => buffer.toSeq :+ -1, // append -1 to output to demonstrate the effect of harvest
@@ -56,7 +56,7 @@ class AggregateWithBoundarySpec extends StreamSpec {
 
     val result = Source(stream)
       .aggregateWithBoundary(allocate = () => ListBuffer.empty[Int])(aggregate = (buffer, i) => {
-        buffer.addOne(i)
+        buffer += i
         (buffer, buffer.sum >= weight)
       }, harvest = buffer => buffer.toSeq, emitOnTimer = None)
       .runWith(Sink.collection)
@@ -145,7 +145,7 @@ class AggregateWithTimeBoundaryAndSimulatedTimeSpec extends AnyWordSpecLike with
       .fromPublisher(p)
       .aggregateWithTimeBoundary(allocate = ListBuffer.empty[Int])(
         aggregate = (buffer, i) => {
-          buffer.addOne(i)
+          buffer += i
           (buffer, false)
         },
         harvest = seq => seq.toSeq,
@@ -189,7 +189,7 @@ class AggregateWithTimeBoundaryAndSimulatedTimeSpec extends AnyWordSpecLike with
       .fromPublisher(p)
       .aggregateWithTimeBoundary(allocate = ListBuffer.empty[Int])(
         aggregate = (buffer, i) => {
-          buffer.addOne(i)
+          buffer += i
           (buffer, false)
         },
         harvest = seq => seq.toSeq,
@@ -230,7 +230,7 @@ class AggregateWithTimeBoundaryAndSimulatedTimeSpec extends AnyWordSpecLike with
       .fromPublisher(upstream)
       .aggregateWithTimeBoundary(allocate = ListBuffer.empty[Int])(
         aggregate = (buffer, i) => {
-          buffer.addOne(i)
+          buffer += i
           (buffer, false)
         },
         harvest = buffer => buffer.toSeq,