National Insurance contribution Demand

Having recently paid the maximum 6 years of additional voluntary contributions to get to full pension, I was slightly perturbed to receive a DEMAND for national insurance for an amount equal to one year? I thought contributions were voluntary. Why have I received a demand? is this normal?

Hi,
You will need to contact our National Insurance team and they will be able to check your National Insurance account and help you further.
National Insurance: general enquiries
Thank you.