Laravel - Dynamic Data & Grammar::columnize() must be of the type array

I have gotten a form of the array to work it seems, though something is still showing up as off when I go to submit the data. Here is my store function:

public function store(Shipment $request)
{

    $this->validate(request(), [

        'pro_number' => 'required',
        'shipment_origin' => 'required'

    ]);

    $user_id = Auth::id();

    $input = $request->all();

    //Save Initial Shipment Data
    $shipment = new Shipment();
    $shipment->pro_number = request('pro_number');
    $shipment->shipment_origin = request('shipment_origin');
    $shipment->date = request('date');
    $shipment->due_date = request('due_date');
    $shipment->tractor_id = request('tractor_id');
    $shipment->trailer_id = request('trailer_id');
    $shipment->driver_id = request('driver_id');
    $shipment->notes = request('notes');
    $shipment->shipper_no = request('shipper_no');
    $shipment->ship_to = request('ship_to');
    $shipment->ship_from = request('ship_from');
    $shipment->bill_to = request('bill_to');
    $shipment->bill_type = request('bill_type');
    $shipment->load_date = request('load_date');
    $shipment->shipment_status = 0;
    $shipment->created_by = $user_id;

    $shipment->save();

    //Save Shipment Details
    $data = array();
        foreach ($request->get('shipment_details') as $shipmentDetails){

                $piecesNumber = $shipmentDetails['piecesNumber'];
                $piecesType = $shipmentDetails['piecesType'];
                $rateType = $shipmentDetails['rateType'];
                $charge = $shipmentDetails['charge'];
                $weight = $shipmentDetails['weight'];
                $hazmat = $shipmentDetails['hazmat'];
                $description = $shipmentDetails['description'];

    $shipment->shipment_details()->save($shipment_detail);
        }
    //Return to Register
    return redirect('/shipments');

}

But whenever I go to save it, I get the following response:

Type error: Argument 1 passed to Illuminate\Database\Grammar::columnize() must be of the type array, string given, called in ...\truckin\vendor\laravel\framework\src\Illuminate\Database\Query\Grammars\Grammar.php on line 131

I believe the error originates somewhere in my saving of shipment_details, though I'm not entirely certain where. It's likely I'm just overlooking something simple, this is new territory to me and I haven't quite gotten a grasp of this stuff yet. If you have any thoughts on this, I'd appreciate the help.

via Chebli Mohamed