In fall 2016, Heechul and Buzz member Min Kyung-hoon released a duet titled "Sweet Dream". The two stars are known as "Universe Cowards" when they promote music, which is a name they came up with for a "Knowing Bros" project group. The song proved successful, topping the South Korean Gaon Digital Chart an achievement known as "all-kill", making him the first Super Junior member to achieve "all-kill" in 2016. In 2017, "Sweet Dream" won a Best Rock Song at the MelOn Music Awards.