A table with variable-length PRIMARY KEY is unaccessible after instant DROP COLUMN

diff --git a/storage/innobase/btr/btr0cur.cc b/storage/innobase/btr/btr0cur.cc

index 9e951f8f78b..5fe71a5eb4d 100644

--- a/storage/innobase/btr/btr0cur.cc

+++ b/storage/innobase/btr/btr0cur.cc

@@ -486,7 +486,7 @@ static dberr_t btr_cur_instant_init_low(dict_index_t* index, mtr_t* mtr)

 			always written with zero length. The DB_TRX_ID will

 			start right after any fixed-length columns. */

 			for (uint i = index->n_uniq; i--; ) {

-				trx_id_offset += index->fields[0].fixed_len;

+				trx_id_offset += index->fields[i].fixed_len;

 			}

 		}

diff --git a/mysql-test/suite/innodb/t/instant_drop.test b/mysql-test/suite/innodb/t/instant_drop.test

index cb1aabce94b..566ac02b314 100644

--- a/mysql-test/suite/innodb/t/instant_drop.test

+++ b/mysql-test/suite/innodb/t/instant_drop.test

@@ -80,6 +80,10 @@ create table t3(f1 int, f2 int not null)engine=innodb;

 insert into t3 values(1, 2);

 alter table t3 drop column f2, add column f3 int default 1, add column f4 int default 4, algorithm=instant;

+create table t4(a varchar(1), b int, c int, primary key(a,b))engine=innodb;

+insert into t4 values('4',5,6);

+alter table t4 drop column c;

+

 --source include/restart_mysqld.inc

 select * from t1;

 alter table t1 add column f6 int default 9,drop column f5, algorithm = instant;

@@ -94,6 +98,11 @@ show create table t2;

 select * from t3;

 alter table t3 add column f5 char(100) default repeat('a', 99), algorithm=instant;

+

+select * from t4;

+alter table t4 add column d varchar(5) default 'fubar';

+insert into t4 values('',0,'snafu');

 --source include/restart_mysqld.inc

 select * from t3;

-drop table t1,t2,t3;

+select * from t4;

+drop table t1,t2,t3,t4;