diff -U3 /home/postgres/pgsql/src/test/regress/expected/strings.out /home/postgres/pgsql/src/test/regress/results/strings.out
--- /home/postgres/pgsql/src/test/regress/expected/strings.out	2021-02-18 10:34:58.190304138 -0500
+++ /home/postgres/pgsql/src/test/regress/results/strings.out	2021-03-21 16:27:22.029402834 -0400
@@ -1443,10 +1443,10 @@
 -- If start plus length is > string length, the result is truncated to
 -- string length
 SELECT substr(f1, 99995, 10) from toasttest;
- substr 
---------
- 567890
- 567890
+         substr         
+------------------------
+ 567890\x7F\x7F\x7F\x7F
+ 567890\x7F\x7F\x7F\x7F
  567890
  567890
 (4 rows)
@@ -1520,10 +1520,10 @@
 -- If start plus length is > string length, the result is truncated to
 -- string length
 SELECT substr(f1, 99995, 10) from toasttest;
- substr 
---------
- 567890
- 567890
+         substr         
+------------------------
+ 567890\177\177\177\177
+ 567890\177\177\177\177
  567890
  567890
 (4 rows)