Where does the Libertarian argument that universal healthcare is akin to slavery come from?

I've seen many Libertarians (even ones such as Rand Paul) claim that universal healthcare is akin to slavery, as you are receiving a Doctor's or a Nurse's services free of charge. However, the fact that the government pays the Doctor's salary in this case clearly negates the analogy to slavery.

I think it's extremely hyperbolic. The public sector Doctor receives all the same benefits as the private sector Doctor. They receive a salary, holidays, pension plans, promotions, etc etc.

Even when I was a Libertarian (now more Liberal) this argument made no sense to me.