diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 0755be8390..5a17112c91 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -2132,9 +2132,11 @@ heapam_scan_bitmap_next_block(TableScanDesc scan,
 	 * Ignore any claimed entries past what we think is the end of the
 	 * relation. It may have been extended after the start of our scan (we
 	 * only hold an AccessShareLock, and it could be inserts from this
-	 * backend).
+	 * backend).  We don't take this optimization in SERIALIZABLE isolation
+	 * though, as we need to examine all invisible tuples reachable by the
+	 * index.
 	 */
-	if (block >= hscan->rs_nblocks)
+	if (!IsolationIsSerializable() && block >= hscan->rs_nblocks)
 		return false;
 
 	/*