key-value and json arrays: {closure}(): Argument #1 ($value) must be of type ?string, array given

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Project extends Model
{
    use HasFactory;

    protected $fillable = ["fm", "content"];
    protected $casts = [
        "fm" => "array"
    ];


    public function getFmAttribute($value)
    {
        $fm = json_decode($value, true);

        if($tags = data_get($fm, 'tags')) {
            $fm['tags'] = implode(',', $tags);
        }

        return $fm;
    }

    public function setFmAttribute($value)
    {
        if($tags = data_get($value, 'tags')) {
            $value['tags'] = explode(',', $tags);
        }
        // dd($value);
        return $value;
    }
}