Does the sun set in a murky pool in Qur'an 18:86?

Non Muslims say that 18:86 is in error. How do we refute this? > Until, when he reached the setting of the sun, he found it [as if] setting in a spring of dark mud, and he found near it a people. Allah said, "O Dhul-Qarnayn, either you punish [them] or else adopt among them [a way of] goodness." -- Qur'an 18:86