Eloquent manyToMany relationship fails on SQL

I have been struggling to make this happen, but every time I failed.

I have two models - Map and User, and both can have many Maps (or users, respectively). The thing is, map has a primary key slug, while User has primary key id and I want to create a pivot table that connects these two models. Here is what I got so far:

Schemes:

Schema::create('users', function (Blueprint $table) {
    $table->id();
    $table->string('name');
    $table->string('email')->unique();
    $table->string('password')->nullable();
    $table->enum('role', \App\Enum\Role::toArray());
    $table->timestamps();
});

Schema::create('maps', function (Blueprint $table) {
    $table->string('name');
    $table->string('slug')->primary();
    $table->string('perex')->nullable();
    $table->string('categories')->nullable();
    $table->enum('company', \App\Enum\Company::toArray());
    $table->string('icon')->nullable();
    $table->boolean('is_public')->default(false);
    $table->boolean('bounds')->default(false);
    $table->timestamps();
});

Schema::create('map_user', function (Blueprint $table) {
    $table->foreignId('user_id')->constrained();
    $table->string('map_slug');

    $table->foreign('map_slug')->references('slug')->on('maps');
});

Map.php

class Map extends Model
{
    protected $primaryKey = 'slug';

    protected $keyType = 'string';

    protected $fillable = [
        'name',
        'slug',
        'perex',
        'categories',
        'company',
        'icon',
        'is_public',
        'bounds',
    ];

    public function suppliers(): Collection
    {
        return $this->hasMany(Supplier::class)->get();
    }

    public function users(): BelongsToMany
    {
        return $this->belongsToMany(User::class, 'map_user', 'map_slug', 'user_id');
    }
}

User.php

class User extends Model
{
    use HasFactory;

    protected $fillable = [
        'name',
        'email',
        'password',
    ];

    public function maps(): BelongsToMany
    {
        return $this->belongsToMany(Map::class, 'map_user', 'user_id', 'map_slug');
    }
}

When creating a new Map, I want to attach the user to it:

/**
 * Store a newly created resource in storage.
*/
public function store(StoreMapRequest $request): RedirectResponse
{
   $user = User::where('email', auth()->user()->getAuthIdentifier())->firstOrFail();

   $map = Map::create([
      'name' => $request->input('name'),
      'slug' => $request->input('slug'),
      'perex' => $request->input('perex'),
      'categories' => $request->input('categories'),
      'company' => $request->enum('company', Company::class),
      'icon' => $request->input('icon'),
      'is_public' => $request->boolean('isPublic'),
      'bounds' => $request->boolean('bounds'),
   ]);

   $map->users()->attach($user->id);

   return to_route('admin.dashboard')->with('success', 'Map created!');
}

and on the ->attach() method I get following error:

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`mapy-dek`.`map_user`, CONSTRAINT `map_user_map_slug_foreign` FOREIGN KEY (`map_slug`) REFERENCES `maps` (`slug`)) 

insert into
  `map_user` (`map_slug`, `user_id`)
values
  (0, 1)

Thank you for your help!