UnexpectedValueException in Response.php line 403 - Laravel 5.1

I am new to Laravel and am building a small project as my schooling. I have struck the above error and I cannot seem to fix it. Any help is appreciated.

In my controller I make the following call when first loading the page. The getDetails method works perfectly on page load, but fails with above error when I call it with Ajax as the user interacts with the page after load.

On page load the method is called and passed to it resulting values from earlier other method calls ($CSpec[0]->id and $CSize[0]->size_mm, which I can see as values 1 and 0.5) as follows:

    $CD = CS::getDetails($CSpec[0]->id, $CSize[0]->size_mm);

Also in the controller I have the following function which triggered via a route used in an Ajax call:

public function itemDetails($ct, $cs)
{
    return CS::getDetails($ct, $s);
}

The getDetails method looks like this:

public static function getDetails($ct, $cs)
{
    return DB::table('recommend_view')
        ->where('recommend_view.id', '=', $ct)
        ->where('recommend_view.size_mm', '=', $cs)
        ->first();
}

I have this route:

Route::get('vd/cd/{cd}/{cs}',
        ['uses' => 'Application\VDController@itemDetails', 'as' => 'application.vdcdetails']);

When I use Ajax to call the same method it fails with the above error. The Ajax code produces the following url in the $.getJSON call:

http://ift.tt/1QM5Ddz

If I dd() inside getDetails for each variable I can see the correct values passed to the method from the url / controller.

So the method works perfectly when called on initial page load with variables fed to it directly from other method calls, but when the variables are passed via the URL / Ajax it fails and I cannot seem to understand what the difference is.

UnexpectedValueException in Response.php line 403:
The Response content must be a string or object implementing __toString(), "object" given.

via Chebli Mohamed