Conversion to utf8mb4 and replication

Hi all,

I’ve got a very strange problem here.

So, there is a database which contains a single table called “questions”:

| Field | Type | Null | Key | Default | Extra |
| id | bigint(11) | NO | PRI | NULL | |
| question_text | varchar(300) | NO | | NULL | |

I am trying to convert it from standard utf8 to utf8mb4. In order to reduce downtime, I am trying to do it in the following way: [LIST=1]
[]Create replica using innobackupex
]Stop replication
[*]Launch pt-online-schema-change on a slave:

pt-online-schema-change --nodrop-old-table --nodrop-new-table --execute --alter "CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci" "D=questions,t=questions"

[]Start replication and wait till it catches the master
]Set master as read-only
[*]Stop replication and promote slave to be a new master
[/LIST] Everything works perfectly till the 4th step. If I run “start slave” after pt-online-schema-change, replication fails with the following error:

.questions' cannot be converted from type 'varchar(900)' to type 'varchar(300)', Error_code: 1677
140717 15:24:41 [ERROR] Error running query, slave SQL thread aborted. Fix the problem, and restart the slave SQL thread with "SLAVE START". We stopped at log 'mysql-bin.004315' position 28070522

Percona MySQL Server version: 5.5.16

Any ideas why is it failing after conversion to utf8mb4?