Go back to immigration to have it fixed. Advise is always to check your passport when you receive it back and ask questions immediately if you are not sure.
I am not not complaining. I had to travel twice to Laos and back in the past to get a non-b so I have wasted all that time and money with the appointments, stress, my time and money travelling there and back etc etc. where I could have done it here. Was only told today that it is possible to do at my local immigration :-) and thanks to this group I know it is true.