For NASA, March 29th was going to be a history-making day at the International Space Station, with the first all-female spacewalk in human history. Astronauts Anne McClain and Christina Koch were scheduled to perform a variety of tasks on the exterior of the space station and it would have been the first time that two female astronauts carried out the delicate work in tandem.

Now that history-making event has been cancelled, and it’s all due to the fit of the spacesuits.

In a new update, NASA reveals that a change has been made to the spacewalk lineup and Koch will now be joined by Nick Hague, who just completed a similar spacewalk on March 22nd.

Concerns over the fit of the spacesuits — more specifically, the fact that only one medium-sized torso component will be available for the spacewalk — have led to McClain being pushed from the lineup. Here’s how NASA explains things:

Koch had been scheduled to conduct this spacewalk with astronaut McClain, in what would have been the first all-female spacewalk. However, after consulting with McClain and Hague following the first spacewalk, mission managers decided to adjust the assignments, due in part to spacesuit availability on the station. McClain learned during her first spacewalk that a medium-size hard upper torso – essentially the shirt of the spacesuit – fits her best. Because only one medium-size torso can be made ready by Friday, March 29, Koch will wear it.

This is obviously a rather disappointing development but, as NASA has warned in the weeks leading up to the spacewalk, plans often change rapidly. In this case, NASA doesn’t want to put McClain in a spacesuit that doesn’t fit her well for the sole purpose of hitting the “first all-female spacewalk” milestone.

When the schedule for these spacewalks was first announced and it looked like it would result in the first all-female spacewalk, NASA was quick to point out that, while it would be a great day for the space agency, the pairing of astronauts isn’t made according to gender. In this case, a last-second change has derailed things a bit, but with NASA sending more female astronauts to the ISS than ever before, it likely won’t be long until they get another shot at making history.