Laravel unable to set foreign key to NULL

Question

I am trying to set a foreign key to null, deleted_by does not allow me to save null as the default value.

I want to be able to set the deleted_by field to null when I create a product. I'm currently using a factory to generate the product but I am unable to seed my database as I can't set the field to null.

Product table

Schema::create('products', function (Blueprint $table) {
    $table->id();
    $table->string('title', 2000);
    $table->string('slug', 2000);
    $table->string('image', 2000)->nullable();
    $table->string('image_mime')->nullable();
    $table->integer('image_size')->nullable();
    $table->longText('description')->nullable();
    $table->decimal('price', 10, 2);
    $table->foreignId('created_by')->references('id')->on('users')->nullable();
    $table->foreignId('updated_by')->references('id')->on('users')->nullable();
    $table->softDeletes();
    $table->foreignId('deleted_by')->references('id')->on('users')->nullable()->constrained();
    $table->timestamps();
});

User table

Schema::create('users', function (Blueprint $table) {
    $table->id();
    $table->string('name');
    $table->string('email')->unique();
    $table->timestamp('email_verified_at')->nullable();
    $table->string('password');
    $table->rememberToken();
    $table->timestamps();
});

Product factory

public function definition()
{
    return [
        'title' => fake()->text(),
        'image' =>fake()->imageUrl(),
        'description' => fake()->realText(200),
        'price' => fake()->randomFloat(2, 2, 5),
        'created_at' => now(),
        'updated_at' => now(),
        'created_by' => 1,
        'updated_by' => 1,
        'deleted_by' => NULL,
    ];
}

Error I am getting

PDOException::("SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'deleted_by' cannot be null")

I don't know why it is not letting me set the default value to null when I'm setting the field as nullable.