diff --git a/Entities/Customer.php b/Entities/Customer.php
index eb6e050..fe5e1ca 100644
--- a/Entities/Customer.php
+++ b/Entities/Customer.php
@@ -59,6 +59,7 @@ class Customer extends BaseCustomer {
 			::where('id', '=', $id)
 			->whereIn('mailbox_id', $mailboxes)
 			->first($columns);
+		return $customer;
 	}
 
 	/**
diff --git a/Providers/MMFRestrictedCustomersServiceProvider.php b/Providers/MMFRestrictedCustomersServiceProvider.php
index 5ce18dc..93bf992 100644
--- a/Providers/MMFRestrictedCustomersServiceProvider.php
+++ b/Providers/MMFRestrictedCustomersServiceProvider.php
@@ -15,6 +15,7 @@ use Illuminate\Support\ServiceProvider;
 use App\User;
 
 use Modules\MMFRestrictedCustomers\Console\Commands\FetchEmails;
+use Modules\MMFRestrictedCustomers\Entities\Customer;
 
 class MMFRestrictedCustomersServiceProvider extends ServiceProvider {
 	/**
@@ -69,6 +70,12 @@ class MMFRestrictedCustomersServiceProvider extends ServiceProvider {
 				if ( array_key_exists('mailbox_id', $data) ) {
 					$customer->mailbox_id = $data['mailbox_id'];
 				} else {
+					$customer = Customer::find($customer->id);
+					if ( $customer == null ) {
+						Log::error('Trying to use a Customer in another Mailbox than the one it is linked to is not allowed, and has been aborted.');
+						// TODO: Error out more gracefully.
+						abort(500, 'Trying to use a Customer in another Mailbox than the one it is linked to is not allowed, and has been aborted.');
+					}
 					if ( $customer->mailbox_id == null ) {
 						Log::error('The creation of a Customer with no link to a Mailbox is not allowed, and has been aborted.');
 						// TODO: Error out more gracefully.