Combo Box Error - Help Please...

I have 5 table (See attachment).

I also have a booking form, which is a combination of the other four tables. The client table is linked to the driver table, as one client is tutored by one driver. So i have created a combo box for both the client and driver, so when you select the client, you can only select the driver who is assigned to that client.

To do this i had to change the row source of the client combo to:
SELECT Client.DriverID, Client.ClientID, Client.Forename, Client.Surname FROM Client;

And create an After Update, Event procedure:
Private Sub cboClient_AfterUpdate()
Me.cboDriver.Requery
End Sub

I changed the row source of the driver combo to:
SELECT Driver.DriverID, Driver.Forename, Driver.Surname FROM Driver WHERE [DriverID]=[cboClient];

This works fine. I can select the client and the driver combo produces the result i want.

The Problem: I get this error msg- You cannot add or change a record because a related record is required in table 'Client'.

The client table contains both the ClientID and the DriverID, so there is a related record. Since the drop down list for the client combo, is related to the client table, i cant see why there is a problem. I hope someone can help, thank you in advance...