[4][5] Her fourth single "Heroine" was released on January 18, 2018 and peaked at number 2 on Gaon Digital Chart.

In response to the reports, her agency MAKEUS Entertainment confirmed that she is preparing for a comeback aimed for September.

[8] On August 20, Sunmi's agency revealed the first teaser for her new album Warning on Twitter along with the release date set for September 4.

On August 26, the tracklist of Warning was released via official social media channels of Make Us Entertainment.

[11] In support of the EP, Sunmi embarked on her first solo world tour visiting cities in North America, Asia and Europe.