Relationships not working (LogicException?)

Hello. I try to build a relationship between my tables users (original from laravel) and collectionson Laravel 5.6.

Schema collections:

        Schema::create('collections', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name');
            $table->string('location');
            $table->integer('user_id')->unsigned()->nullable();
            $table->foreign('user_id')->references('id')->on('users')->onDelete('set null');
            $table->timestamps();
        });

The Collection model:

    public function user() {
        $this->belongsTo('\App\User');
    }

The User model:

    public function collection() {
        $this->hasMany('\App\Collection');
    }

I try to get the user->name using artisan tinker:

>>>  $collection->user()->get();
PHP Error:  Call to a member function get() on null on line 1
>>>  $collection->user()->name;
PHP Notice:  Trying to get property of non-object on line 1
>>> $collection->user->name;
LogicException with message 'App\Collection::user must return a relationship instance.'

Where is my mistake? Thanks!