Q: iPhone 6s charger "not certified"

Hi,

When I plugged in my iPhone 6s this afternoon, it came up with a message, in short, saying that the charger was "not certified". This had never happened to me before, so I thought it was odd. I plugged in my charger for my iPad, and it worked. This means that it was not a problem with the charging port, more with the charger. Does anyone have any ideas as to what it might be?

Thanks