Heavyweight champion Deontay Wilder can choose his next opponent in defending his World Boxing Council title.

Wilder was scheduled to fight mandatory challenger Alexander Povetkin on May 21. But Povetkin failed a drug test, which showed the banned substance meldonium, and the bout was canceled.

While the WBC investigates Potemkin, Wilder has approval from the organization for a voluntary defense of the belt.

Povetkin passed at least two tests before failing one in April. Meldonium is a stamina booster that tennis star Maria Sharapova and dozens of other athletes in ex-Soviet republics have tested positive for since it was banned Jan. 1.