Cannot add a column to an InnoDB table in Mariadb

I have this InnoDB table with less than 10 rows in mysql 5.5.40-MariaDB on Linux (x86_64).

+-----------------+--------------+------+-----+-------------------+----------------+
| Field           | Type         | Null | Key | Default           | Extra          |
+-----------------+--------------+------+-----+-------------------+----------------+
| id              | int(11)      | NO   | PRI | NULL              | auto_increment |
| current_version | varchar(20)  | NO   |     | NULL              |                |
| last_updated    | timestamp    | NO   |     | CURRENT_TIMESTAMP |                |
| key             | varchar(256) | NO   |     | NULL              |                |
| language        | varchar(20)  | NO   |     | NULL              |                |
| text            | text         | NO   |     | NULL              |                |
+-----------------+--------------+------+-----+-------------------+----------------+

When I try to add a column, the client hangs like so:

MariaDB [pkppln]> alter table terms_of_use add column `weight` int(11) not null default 0;
Stage: 2 of 2 'Enabling keys'      0% of stage done

The server continues to respond to other queries, but the client does nothing. I can kill the query with ctrl-C. The alter table command works in my dev environment, but seems to be failing here.

How can I fix this, and add the column?