Laravel PHPUnit在null上调用成员函数connection()

我正在学习Laravel上的测试，并且由于此错误，我无法继续测试Eloquent:

I am learning testing on Laravel and I am unable to continue testing Eloquent by this error:

错误:在null上调用成员函数connection()

Error: Call to a member function connection() on null

这是我的代码:

public function test_users_can_follow_each_other()
    {
        $this->userOne = User::find(1);
        $this->userTwo = User::find(2);
        $this->userOne->followedBy($this->userTwo->id);
        $this->assertTrue($this->userTwo->isFollowedBy($this->userOne->id));
    }

我也在使用特征 use DatabaseTransactions;

该数据库已经在users表上创建了2条记录.我还需要配置其他内容吗，谢谢！

The database is already created with 2 records on users table. Do I need to config anything else, thanks!