Laravel 5 Querying "Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_ERROR) Call to a member function addEagerConstraints() on null"

Question

Trying to establish different relationships between serial number model and warranties

    public function serialNumber()
    {
        return $this->belongsTo(SerialNumber::class, 'serial_no');
    }
    
    public function warranty()
        {
            $serialNumber = $this->serialNumber;
    
            if ($serialNumber) {
                return $serialNumber->warranty ?? null;
            }
    
            return null;
        }

It didnt fetch