Interest not supposedly charged in January

I settled my blue bean credit card in February and closed the Account the same month on the 16 this year. Now on 28 April 2015. They send me a statement saying I owe money for interest not charged in January. When I ask them why was I not notified about this when I settled and closed the account, They say they apologise for the inconvinience but I am liable... Now tell me why should I pay for the they made and they aknowledge that error?<br> <br> Please help me I do not think i owe Standard Bank anything hence I settled and closed the Account in February 2015.