Laravel 5.1 find eloquent returns null

I've this model Customer :

<?php
 namespace App\Models;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\softDeletes;

   class Customer extends Model
   {
        use SoftDeletes;
        protected $table = 'customers';
        public $timestamps = true;
        protected $dates = ['deleted_at'];
        protected $fillable = ['name', ...
        ...

What I can't understand is : When I try this :

$customer = Customer::all();                
dd($customer);

I get results

and when trying this :

$customer = Customer::find(1);                
dd($customer);

I get null.

via Chebli Mohamed