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..c674e78a31 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
@@ -211,7 +211,6 @@ import pekko.util.ccompat.JavaConverters._
       tailSource.setHandler(subHandler)
       setKeepGoing(true)
       scheduleOnce(SubscriptionTimer, timeout)
-      builder = null
       Source.fromGraph(tailSource.source)
     }
 
@@ -222,13 +221,16 @@ import pekko.util.ccompat.JavaConverters._
         builder += grab(in)
         left -= 1
         if (left == 0) {
-          push(out, (builder.result(), openSubstream()))
+          val prefix = builder.result()
+          builder = null // free for GC
+          push(out, (prefix, openSubstream()))
           complete(out)
         } else pull(in)
       }
     }
     override def onPull(): Unit = {
       if (left == 0) {
+        builder = null // free for GC
         push(out, (Nil, openSubstream()))
         complete(out)
       } else pull(in)
@@ -237,7 +239,9 @@ 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()