Your salary is at a level (£680 per month for 2017/18) that is high enough to count towards your state pension contributions but low enough so that there are no NI contributions to be made.  You might assume that you need to pay NI in order to keep your state pension entitlement but that is not the case.

(This means you can't take advantage of the employer's national insurance allowance, but the savings are greater by keeping your salary at £680 per month).