Fabricio Werdum has spoken out following his U.S. Anti-Doping Agency (USADA) flag.

Werdum was scheduled to take on Aleksei Oleinik at UFC Moscow in September. Those plans appear to have been quickly derailed after “Vai Cavalo” was flagged for a potential violation of the anti-doping policy. The news comes just after Werdum was rewarded with a 25x shirt for testing clean 27 times.

The former UFC heavyweight champion took to Instagram to release the following statement:

“Hey guys, I got up this morning to the news that one of my urine samples from April tested positive for a prohibited substance. I am working with my team, the UFC and USADA to understand what happened. I’ve always been careful with everything I take and I’ve always supported a clean sport. We will work hard to solve this misunderstanding and I hope soon to be able to go back to the octagon and do what I love.”

Werdum was hoping to get back in the win column against Oleinik. “Vai Cavalo” last competed against Alexander Volkov in the main event of UFC London. Werdum ended up being knocked out in the fourth round. He had won back-to-back bouts going into his tilt with Volkov.

“Vai Cavalo” won the UFC heavyweight title back in June 2015. Werdum submitted Cain Velasquez in Mexico City to capture the gold. In his first title defense, Werdum was knocked out by Stipe Miocic in the first round.

Werdum was able to rebound with a unanimous decision victory over Travis Browne. He then fell short in the rubber match with Alistair Overeem. The former champion was able to nab a quick submission win over Walt Harris, followed by a dominant decision victory over Marcin Tybura. He has gone 3-2 in his last five outings.