--- instant_alter_limit.result	2020-05-26 18:01:27.377946439 +0530
+++ instant_alter_limit,64k.reject	2020-05-26 20:00:22.499711222 +0530
@@ -43,5 +43,12 @@
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-502
+506
 DROP TABLE t;
+#
+# MDEV-21787 Alter table failure tries to access uninitialized column
+#
+CREATE TABLE t1(f1 INT PRIMARY KEY, f2 TEXT GENERATED ALWAYS AS (SUBSTR(f4, 1, 400)), f3 VARCHAR(500), f4 TEXT)ENGINE=InnoDB ROW_FORMAT=Compact;
+ALTER TABLE t1 ADD UNIQUE KEY (f2(9));
+ALTER TABLE t1 ADD COLUMN f5 TEXT, ALGORITHM=INPLACE;
+DROP TABLE t1;