diff --git a/processing/src/main/java/org/apache/druid/frame/processor/SuperSorter.java b/processing/src/main/java/org/apache/druid/frame/processor/SuperSorter.java
index 00be9a3166a7..5cb9c0830e49 100644
--- a/processing/src/main/java/org/apache/druid/frame/processor/SuperSorter.java
+++ b/processing/src/main/java/org/apache/druid/frame/processor/SuperSorter.java
@@ -670,7 +670,7 @@ private void runMerger(
         final OutputChannel outputChannel = outputChannelFactory.openChannel(intRank);
         writableChannel = outputChannel.getWritableChannel();
         frameAllocatorFactory = new SingleMemoryAllocatorFactory(outputChannel.getFrameMemoryAllocator());
-        outputChannels.set(intRank, outputChannel);
+        outputChannels.set(intRank, outputChannel.readOnly());
 
         if (totalMergingLevels == 1) {
           // Reading from the inputBuffer. (i.e. "direct mode"; see class-level javadoc for more details.)