Things are good to go (and the mission patch looks sweet) for SpaceX’s Falcon 9 launch Saturday, except for one little snag: CEO Elon Musk says there might be a little leak.

On Friday, the CEO tweeted that the company was investigating a “very small” leak in the rocket’s upper stage — Musk downplayed any concern over the incident, but if the company is running an investigation, it could definitely be an issue.

The launch window for this ISS resupply mission — the first of a busy 2017 SpaceX launch calendar — opens at 10:01 a.m. on Saturday at Kennedy Space Center’s historic LC-39a launch pad used for the Apollo missions, including the Apollo 11 moon mission, in the 1960s.

The leak is troubling — the fueling process is one of the most dangerous steps in a rocket launch due to how insanely volatile the liquid oxygen used to blast spaceships out of the atmosphere is. And the company is still trying to get back on its feet after a massive accident sent a Falcon 9 up in flames in September. Investigators think the September incident occurred when a support strut collapsed and let some of the explosive fuel escape, setting off a chain reaction fireball.

According to Musk, the current leak is small, so it’s probably a different issue than the one he called the company’s “most complex failure ever.”

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The most recent government report on SpaceX’s progress toward meeting NASA’s Commercial Crew Program requirements — which would send humans to the ISS and relieve NASA of relying on Russia’s Soyuz capsule — raised concerns about the fueling process for Falcon 9 rockets. In the company’s proposed system, the rockets would be fueled with crew inside the capsule already, making it extra-important for the company to iron out any kinks in how propellant gets into the vehicle.

The launch is still on until Musk says otherwise — we’ll update this post when we know more. 

Photos via SpaceX, Getty Images / Bill Pugliano