LONDON (AP) — Ukrainian boxer Oleksandr Usyk is facing a late change of opponent for his first fight at heavyweight.

Usyk’s promoter, Eddie Hearn, says on Twitter that he has been informed by the Voluntary Anti–Doping Association (VADA) that Tyrone Spong has returned an adverse doping test, which has been sent to the State of Illinois commission.

Usyk, who is making the step up to heavyweight after unifying the cruiserweight division in 15 fights, was scheduled to be fighting Spong in Chicago on Saturday.

Hearn says “we have reserve opponents standing by.”

Spong, a Surinamese–Dutch fighter, quit kickboxing for boxing in 2015 and has won all 14 of his fights.

VADA, which runs anti–doping programs in boxing and MMA, said on Sept. 20 that Spong had joined its program ahead of the Usyk fight.

Usyk said in August he’d rejected the options of fighting veteran Russian heavyweights Alexander Povetkin or Andrei Fedosov because he feared it would be seen as a political battle.

Usyk was born on the Crimean peninsula, which was annexed by Russia from Ukraine in 2014, and sees himself as Ukrainian. However, he did fight Russian boxer Murat Gassiev in Moscow at cruiserweight.