diff --git a/server/netty-server/src/main/scala/sttp/tapir/server/netty/internal/reactivestreams/SimpleSubscriber.scala b/server/netty-server/src/main/scala/sttp/tapir/server/netty/internal/reactivestreams/SimpleSubscriber.scala
index 2944f27182..5408274822 100644
--- a/server/netty-server/src/main/scala/sttp/tapir/server/netty/internal/reactivestreams/SimpleSubscriber.scala
+++ b/server/netty-server/src/main/scala/sttp/tapir/server/netty/internal/reactivestreams/SimpleSubscriber.scala
@@ -6,7 +6,6 @@ import org.reactivestreams.{Publisher, Subscription}
 import sttp.capabilities.StreamMaxLengthExceededException
 
 import java.util.concurrent.LinkedBlockingQueue
-import scala.collection.mutable.ListBuffer
 import scala.concurrent.{Future, Promise}
 
 private[netty] class SimpleSubscriber(contentLength: Option[Int]) extends PromisingSubscriber[Array[Byte], HttpContent] {
@@ -14,7 +13,7 @@ private[netty] class SimpleSubscriber(contentLength: Option[Int]) extends Promis
   private val resultPromise = Promise[Array[Byte]]()
   @volatile private var totalLength = 0
   private val resultBlockingQueue = new LinkedBlockingQueue[Either[Throwable, Array[Byte]]](1)
-  @volatile private var buffers = new ListBuffer[ByteBuf]
+  @volatile private var buffers = Vector[ByteBuf]()
 
   override def future: Future[Array[Byte]] = resultPromise.future
   def resultBlocking(): Either[Throwable, Array[Byte]] = resultBlockingQueue.take()
@@ -39,7 +38,7 @@ private[netty] class SimpleSubscriber(contentLength: Option[Int]) extends Promis
         subscription.request(1)
       }
     } else {
-      buffers = buffers.append(byteBuf)
+      buffers = buffers :+ byteBuf
       totalLength += byteBuf.readableBytes()
       subscription.request(1)
     }
@@ -49,7 +48,7 @@ private[netty] class SimpleSubscriber(contentLength: Option[Int]) extends Promis
     buffers.foreach { buf =>
       val _ = buf.release()
     }
-    buffers.clear()
+    buffers = Vector.empty
     resultBlockingQueue.offer(Left(t))
     resultPromise.failure(t)
   }
@@ -64,7 +63,7 @@ private[netty] class SimpleSubscriber(contentLength: Option[Int]) extends Promis
         currentIndex += length
         val _ = buf.release()
       }
-      buffers.clear()
+      buffers = Vector.empty
       if (!resultBlockingQueue.offer(Right(mergedArray))) {
         // Result queue full, which is unexpected.
         resultPromise.failure(new IllegalStateException("Calling onComplete after result was already returned"))