DB Query builder reports column not found, but refers to a value instead of a column title

I have the table:

CREATE TABLE `project_user` (
  `project_id` int(10) unsigned NOT NULL,
  `user_id` int(10) unsigned NOT NULL,
  `role_id` int(11) NOT NULL,
  `created_at` timestamp NULL DEFAULT NULL,
  `updated_at` timestamp NULL DEFAULT NULL,
  KEY `project_user_project_id_index` (`project_id`),
  KEY `project_user_user_id_index` (`user_id`),
  CONSTRAINT `project_user_project_id_foreign` FOREIGN KEY (`project_id`) REFERENCES `projects` (`id`),
  CONSTRAINT `project_user_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

which includes the record

1 1 3 2018-01-03 12:12:01 2018-01-03 12:12:01

and when I run with tinker the query

DB::table('project_user')->whereColumn([['project_id', '=', 1], ['user_id', '=', 1]])->get();

I get the error message

Illuminate\Database\QueryException with message 'SQLSTATE[42S22]: Column not found: 1054 Unknown column '1' in 'where clause' (SQL: select * from `project_user` where (`project_id` = `1` and `user_id` = `1`))'

Its reporting Column not found: 1054 Unknown column '1', ... but 1 is not a column title.