$wpdb->insert does nothing when creating a table programmatically

I am creating a custom table in the WP database. However, after this is created successfully I don’t seem to be able to actually insert a row into it… Am I missing something here?

// store the specified img URL in the wp db
if (!empty($_POST['imgURL'])) {
  global $wpdb;
  $table_name = $wpdb->prefix . 'my_custom_table';

  if ($wpdb->get_var('SHOW TABLES LIKE "' . $table_name . '"') === $table_name) {
    $delete = $wpdb->query('TRUNCATE TABLE ' . $table_name);
  } else {
    require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
    $charset_collate = $wpdb->get_charset_collate();
    $create = "CREATE TABLE $table_name (
      url varchar(55) NOT NULL DEFAULT ''
    ) $charset_collate;";
    dbDelta($create);
  }

  $wpdb->insert($table_name, array('url' => $_POST['imgURL']));    // why doesn't this work???
  wp_send_json_success(array('msg' => 'Image inserted in database successfully'));
}