CAPE CANAVERAL, Fla. (AP) — Boeing called off its first astronaut launch because of a valve problem on the rocket Monday night.

The two NASA test pilots had just strapped into Boeing’s Starliner capsule when the countdown was halted, just two hours before the planned liftoff. A United Launch Alliance engineer, Dillon Rice, said the issue involved an oxygen relief valve on the upper stage of the company’s Atlas rocket.

There was no immediate word on when the team would try again to launch the test pilots to the International Space Station for a weeklong stay. It was the latest delay for Boeing’s first crew flight, on hold for years because of capsule trouble.

“In a situation like this, if we see any data signature is not something that we have seen before, then we are just simply not willing to take any chances with what is our most precious payload,” Rice said.