diff --git a/src/backend/utils/sort/logtape.c b/src/backend/utils/sort/logtape.c
index 6b7c10bcfca..66bfcced8d7 100644
--- a/src/backend/utils/sort/logtape.c
+++ b/src/backend/utils/sort/logtape.c
@@ -86,6 +86,7 @@
 #include "storage/buffile.h"
 #include "utils/builtins.h"
 #include "utils/logtape.h"
+#include "utils/memdebug.h"
 #include "utils/memutils.h"
 
 /*
@@ -874,6 +875,17 @@ LogicalTapeFreeze(LogicalTapeSet *lts, int tapenum, TapeShare *share)
 	 */
 	if (lt->dirty)
 	{
+		/*
+		 * As long as we've filled the buffer at least once, its contents are
+		 * entirely defined from valgrind's point of view, even though
+		 * contents beyond the current end point may be stale.  But it's
+		 * possible - at least in the case of a parallel sort - to sort such
+		 * small amount of data that we do not fill the buffer even once. Tell
+		 * valgrind that its contents are defined, so it doesn't bleat.
+		 */
+		VALGRIND_MAKE_MEM_DEFINED(lt->buffer + lt->nbytes,
+								  lt->buffer_size - lt->nbytes);
+
 		TapeBlockSetNBytes(lt->buffer, lt->nbytes);
 		ltsWriteBlock(lts, lt->curBlockNumber, (void *) lt->buffer);
 		lt->writing = false;