Many to Many Pivot table column names

Hopefully a fairly simple issue and I'm just missing the bleeding obvious.

I have 2 models Clients and Bookings that are Many to Many related via the client_booking pivot table.

My Clients model relationship to bookings is set up as follows:

public function bookings()
    {
        return $this->belongsToMany('Modules\Courses\Entities\Booking', 'courses__booking_client');
    }

and my Bookings model relationship to Clients is set up as follows:

 public function clients()
    {
        return $this->belongsToMany("Modules\Client\Entities\Clients", "courses__booking_client");
    }

My courses__booking_client table just consists of two columns booking_id and client_id

In my controller I can add multiple clients to a booking in the store action, I loop through save the new clients and then attach them to the newly saved booking:

// pseudo code below 
// loop through and create the clients using populated data from form
foreach ($request->clients as $newClient {
    // aggegate the data into an array

    $client = Client::create($data);

    $clientIds[] = $client->id;
}

// create the booking
$booking = Booking::create($bookingData);

// Now associate the clients to the booking
$booking->clients()->attach($clientIds); <-- Uh oh!

Everything works until the uh oh line - clients are created, booking is created but I get an exception thrown:

Column not found: 1054 Unknown column 'clients_id' in 'field list' (SQL: insert into `courses__booking_client` (`booking_id`, `clients_id`) values (9, 21), (9, 22), (9, 8))

Why is eloquent trying to populate a column clients_id instead of client_id. All the docs use singular for the column name. e.g user_id and role_id even though the model relationships are plural https://laravel.com/docs/5.4/eloquent-relationships#many-to-many

And how do i go about rectifying this - is this a bug or do I need to rename my table column to match?

TIA BP