I resigned my position, my health care is still good until t

My previous employer told me I would be covered through the end of the month after I resigned on the 18th. However, a month later they sent me a bill claiming that I did not have enough drawn out to cover me through the end of the month and the employer had covered the costs of my last month; therefore, I now owed my previous employer the last month's premium. Am I liable for this money even though I was told that I was covered through the end of the month?