Error while creating custom module in drupal 8

I am developing custom module in drupal 8 , try to create a database getting

sqlstate error: PHP message: Uncaught PHP Exception DrupalCoreDatabaseDatabaseExceptionWrapper: “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 ‘DEFAULT ‘NULL’,

function zimo_schema() {
  $schema['zimo'] = array(
    'description' => 'Stores example person entries for demonstration purposes.',
    'fields' => array(
      'pid' => array(
        'type' => 'serial',
        'not null' => TRUE,
        'description' => 'Primary Key: Unique person ID.',
       ),
      
      'name' => array(
        'type' => 'varchar',
        'length' => 255,
        'default' => 'NULL',
       ),

      'paiddate' => array(
       'type' => 'date',
       'default' => 'NULL', 
	),
      'amount' => array(
        'type' => 'bigint',
       'default' => 'NULL',
       ),
    ),
    'primary key' => array('pid'),

  );

  return $schema;
}


Please Help me to resolve this

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/4/feed