Confusing error ER_PARTITION_INSTEAD_OF_SUBPARTITION upon trying to convert table to partition

--source include/have_partition.inc

create table t (a int, b int)

partition by range (a)

subpartition by hash(b) subpartitions 2 (

 partition p0 values less than (100)

);

create table t1 (a int, b int);

alter table t convert table t1 to partition p2 values less than (maxvalue);

# Cleanup

drop table if exists t, t1;

query 'alter table t convert table t1 to partition p2 values less than (maxvalue)' failed:

ER_PARTITION_INSTEAD_OF_SUBPARTITION (1734): Subpartitioned table, use subpartition instead of partition