diff --git a/src/backend/access/nbtree/nbtxlog.c b/src/backend/access/nbtree/nbtxlog.c
index 5eca4ab2727..e1c37491485 100644
--- a/src/backend/access/nbtree/nbtxlog.c
+++ b/src/backend/access/nbtree/nbtxlog.c
@@ -281,7 +281,11 @@ btree_xlog_split(bool onleft, XLogReaderState *record)
 			datalen -= newitemsz;
 		}
 
-		/* Extract left hikey and its size (assuming 16-bit alignment) */
+		/*
+		 * Extract left hikey and its size.  We assume that 16-bit alignment
+		 * is enough to apply IndexTupleSize (since it's fetching from a
+		 * uint16 field).
+		 */
 		left_hikey = (IndexTuple) datapos;
 		left_hikeysz = MAXALIGN(IndexTupleSize(left_hikey));
 		datapos += left_hikeysz;