diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 4bc24d9..14e76fe 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -340,9 +340,13 @@ TruncateSUBTRANS(TransactionId oldestXact)
 
 	/*
 	 * The cutoff point is the start of the segment containing oldestXact. We
-	 * pass the *page* containing oldestXact to SimpleLruTruncate.
+	 * pass the *page* containing oldestXact to SimpleLruTruncate.  We step
+	 * back one transaction to avoid passing a cutoff page that hasn't been
+	 * created yet in the rare case that oldestXact would be the first item on
+	 * a page, and oldestXact == next XID (immediately after vacuum has
+	 * advanced oldestXact to point to an XID that hasn't been created yet).
 	 */
-	cutoffPage = TransactionIdToPage(oldestXact);
+	cutoffPage = TransactionIdToPage(oldestXact - 1);
 
 	SimpleLruTruncate(SubTransCtl, cutoffPage);
 }