can't add primary key to mysql table after the table is created

A table in this application had no primary keys. I created an entity
so i could attempt to use the doctrine:schema:update command to fix it up.

When i attempted to add this:

• @ORM\Id
• @ORM\GeneratedValue(strategy="NONE")

to this:

• @ORM\Column(name="session_id", type="string", length=128,
  nullable=false)

it generates SQL like this:

ALTER TABLE whos_online DROP INDEX idx_session_id_zen, ADD UNIQUE INDEX primary (session_id);
ALTER TABLE whos_online CHANGE ip_address ip_address VARCHAR(45) NOT NULL, CHANGE host_address host_address VARCHAR(512) NOT NULL;

and says this:

An exception occurred while executing 'ALTER TABLE whos_online DROP INDEX idx_session_id_zen, ADD UNIQUE INDEX primary (session_id );
SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to
your MySQL server version for the right syntax to use near 'primary (session_id)' at line 1