diff --git a/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/StreamOfStreams.scala b/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/StreamOfStreams.scala
index dc84291b2e..4c9b9cea7b 100644
--- a/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/StreamOfStreams.scala
+++ b/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/StreamOfStreams.scala
@@ -237,7 +237,9 @@ import pekko.util.ccompat.JavaConverters._
     override def onUpstreamFinish(): Unit = {
       if (!prefixComplete) {
         // This handles the unpulled out case as well
-        emit(out, (builder.result(), Source.empty), () => completeStage())
+        val prefix = builder.result();
+        builder = null // free for GC
+        emit(out, (prefix, Source.empty), () => completeStage())
       } else {
         if (!tailSource.isClosed) tailSource.complete()
         completeStage()