`replace into` with global index report error

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a varchar(70), b mediumint(9), unique index idx_a(a), unique index idx_b(b)) partition by key(b) partitions 5;
insert into t values ('',826534 );
replace into t values ('',826536 );

2. What did you expect to see? (Required)

no error

3. What did you see instead (Required)

mysql> replace into t values ('',826536 );
ERROR 8141 (HY000): assertion failed: key: 7480000000000003bb5f6980000000000000020380000000000c9ca6, assertion: Exist, start_ts: 450206017021804545, existing start ts: 0, existing commit ts: 0

4. What is your TiDB version? (Required)