AlterTableAlterPartitionOperation cannot change the type if the column has default partition

create database pt;
create table pt.alterdynamic_part_table(intcol string) partitioned by (partcol1 string, partcol2 string);
insert into table pt.alterdynamic_part_table partition(partcol1, partcol2) select NULL, '2', NULL;
alter table pt.alterdynamic_part_table partition column (partcol2 int);

org.apache.hadoop.hive.ql.metadata.HiveException: Exception while checking type conversion of existing partition values to FieldSchema(name:partcol2, type:int, comment:null) : Exception while converting string to int for value : NULL
    at org.apache.hadoop.hive.ql.ddl.table.partition.alter.AlterTableAlterPartitionOperation.check(AlterTableAlterPartitionOperation.java:69)
    at org.apache.hadoop.hive.ql.ddl.table.partition.alter.AlterTableAlterPartitionOperation.execute(AlterTableAlterPartitionOperation.java:55)