Eloquent Many to Many attach() is passing a null model ID

I am developing an application using Laravel and Eloquent ORM, it uses a database filled with event information.

I have successfully implemented attach() in the relevant controllers for both my user and role models.

My Event model can have many Links. A Link can have Many events.
My problem is that the attach() is not supplying the ID of the object it is being called on, instead it supplies null and I receive the following error message:

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'link_id'
cannot be null (SQL: insert into 'event_link' ('created_at', 'event_id',
'link_id', 'updated_at') values (2018-06-09 11:27:15, 2, , 2018-06-09 11:27:15))

I’ve triple checked my models and database structure.

I can’t even imagine how this error could occur since the id lacking in the SQL query is the id of the object that the attach() method is actually being called on. If I use sync($eventID, false) instead of attach(), the result is the same.

Structure of my three SQL tables

Event table

Link table

Event_Link table

The following is the problematic method in the controller responsible for storing the record and creating an entry in the event_link weak entity.
The $link object is created successfully, if attach() is commented out, a JSON representation of a link is returned (but it lacks the ‘id‘ field).

    public function store(StoreLink $request) {
      $link = Link::create([
        'title' => $request->title,
        'url' => $request->url,
      ]);

      if ($request['eventId']) {
        // $request->eventId is passed successfully, $link id is not passed.
        $link->events()->attach($request->eventId);
      }
      return response()->json($link, 201);
    }

Link Model

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Link extends Model
{
  protected $table = 'links';
  public $incrementing = false;
  public $timestamps = true;
  protected $primaryKey = "id";
  protected $fillable = ['title', 'url'];

  public function events()
  {
    return $this->belongsToMany('App\Event')->withTimestamps();
  }
}

Event Model

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Event extends Model
{
  protected $table = 'events';
  public $incrementing = false;
  public $timestamps = true;
  protected $primaryKey = "id";
  protected $fillable = ['description', 'date', 'image', 'category_id'];

  public function category()
  {
      return $this->belongsTo('App\Event', 'category_id');
  }

  public function links()
  {
    return $this->belongsToMany('App\Link', 'event_link')->withTimestamps();
  }

  public function history()
  {
    return $this->belongsToMany('App\User', 'user_history');
  }

  public function favourites()
  {
    return $this->belongsToMany('App\User', 'user_favourites');
  }

}