问Drupal 7更新架构中断

EN

function license_update_7101() {

  $schema['license_agreements'] = array(
    'description' => 'License User Agreements',
    'fields' => array(
      'license_id' => array(
        'description' => 'Primary key for license records',
        'type'        => 'serial',
        'not null'    => TRUE,
        'unsigned'    => TRUE
      ),
      'user_id' => array(
        'description' => 'License agreed to by User ID',
        'type'        => 'int',
        'not null'    => FALSE,
        'unsigned'    => TRUE,
        'default'     => '0'
      ),
      'product_id' => array(
        'description' => 'Product ID that represents the product this agreement was assigned to.',
        'type'        => 'int',
        'not null'    => FALSE,
        'unsigned'    => TRUE,
        'default'     => '0'
      ),
      'session' => array(
        'description' => 'Session created during account creation.',
        'type'        => 'varchar',
        'length'      => 32,
        'not null'    => FALSE,
        'default'     => ''
      ),
      'agreed' => array(
        'description' => 'Represents if the license was agreed to or not',
        'type'        => 'int',
        'size'        => 'tiny',
        'not null'    => FALSE,
        'unsigned'    => TRUE,
        'default'     => '0'
      ),
      'agreement_date' => array(
        'description' => 'Date this license was agreed to by user.',
        'type'        => 'datetime',
        'mysql_type'  => 'DATETIME',
        'disp-width'  => '11',
        'not null'    => FALSE,
        'default'     => NULL
      ),
      'blog_name' => array(
        'description' => 'URL for actual product license belongs to.',
        'type'        => 'varchar',
        'length'      => '128',
        'not null'    => FALSE,
        'default'     => ''
      ),
      'primary key' => array('license_id')
    )
  );

  db_create_table('license_agreements', $schema['license_agreements']);
}

SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MariaDB    [error]
server version for the right syntax to use near 'DEFAULT NULL
) ENGINE = InnoDB DEFAULT CHARACTER SET utf8 COMMENT 'License' at line 9