diff --git a/OpenActive.Server.NET/CustomBookingEngine/CustomBookingEngine.cs b/OpenActive.Server.NET/CustomBookingEngine/CustomBookingEngine.cs
index a5902b84..a8818233 100644
--- a/OpenActive.Server.NET/CustomBookingEngine/CustomBookingEngine.cs
+++ b/OpenActive.Server.NET/CustomBookingEngine/CustomBookingEngine.cs
@@ -756,11 +756,16 @@ async Task<ResponseContent> IBookingEngine.TriggerTestAction(string actionJson)
         //TODO: Should we move Seller into the Abstract level? Perhaps too much complexity
         protected BookingFlowContext ValidateFlowRequest<TOrder>(OrderIdComponents orderId, SellerIdComponents sellerIdComponents, ILegalEntity seller, FlowStage stage, TOrder order) where TOrder : Order, new()
         {
-            if (order?.Seller.IdReference != null && seller?.Id != order?.Seller.IdReference)
+            if (order?.Seller.IdReference == null)
             {
                 throw new OpenBookingException(new SellerMismatchError());
             }
 
+            if (seller?.Id != order?.Seller.IdReference)
+            {
+                throw new OpenBookingException(new InvalidAuthorizationDetailsError());
+            }
+
             // Check that taxMode is set in Seller
             if (!(seller?.TaxMode == TaxMode.TaxGross || seller?.TaxMode == TaxMode.TaxNet))
             {