Ajax post failing with TypeError

I have defined the route and when I actually fire the JS I end up with the following: (CI 4.6.0, on FireFox). Is this a CI problem please?
{
    "title": "TypeError",
    "type": "TypeError",
    "code": 500,
    "message": "CodeIgniter\\HTTP\\Request::fetchGlobal(): Argument #3 ($filter) must be of type ?int, true given, called in /var/www/html/gnb/ci460/system/HTTP/IncomingRequest.php on line 714",
    "file": "/var/www/html/gnb/ci460/system/HTTP/RequestTrait.php",
    "line": 258,
    "trace": [
        {
            "file": "/var/www/html/gnb/ci460/system/HTTP/IncomingRequest.php",
            "line": 714,
            "function": "fetchGlobal",
            "class": "CodeIgniter\\HTTP\\Request",
            "type": "->"
        },
        {
            "file": "/var/www/html/gnb/ci460/app/Controllers/TheBigPicture.php",
            "line": 148,
            "function": "getPost",
            "class": "CodeIgniter\\HTTP\\IncomingRequest",
            "type": "->"
        },
        {
            "file": "/var/www/html/gnb/ci460/system/CodeIgniter.php",
            "line": 933,
            "function": "interact",
            "class": "App\\Controllers\\TheBigPicture",
            "type": "->"
        },
        {
            "file": "/var/www/html/gnb/ci460/system/CodeIgniter.php",
            "line": 507,
            "function": "runController",
            "class": "CodeIgniter\\CodeIgniter",
            "type": "->"
        },
        {
            "file": "/var/www/html/gnb/ci460/system/CodeIgniter.php",
            "line": 354,
            "function": "handleRequest",
            "class": "CodeIgniter\\CodeIgniter",
            "type": "->"
        },
        {
            "file": "/var/www/html/gnb/ci460/system/Boot.php",
            "line": 334,
            "function": "run",
            "class": "CodeIgniter\\CodeIgniter",
            "type": "->"
        },
        {
            "file": "/var/www/html/gnb/ci460/system/Boot.php",
            "line": 67,
            "function": "runCodeIgniter",
            "class": "CodeIgniter\\Boot",
            "type": "::"
        },
        {
            "file": "/var/www/html/gnb/ci460/public/index.php",
            "line": 56,
            "function": "bootWeb",
            "class": "CodeIgniter\\Boot",
            "type": "::"
        }
    ]
}

Can you share the code of TheBigPicture controller?