As Fallout: London, a huge mod for Fallout 4, is preparing to launch. Distributor GOG has clarified some questions about compatibility. The company confirmed that Fallout: London will work with versions of Fallout 4 purchased on GOG and Steam, but not with those purchased on the Epic Games Store, as the platform does not currently support a feature needed to run mods.

If you haven’t been up to date on its progress, Fallout: London – which, as its name suggests, allows players to explore an open-world version of post-apocalyptic London – has hit a few bumps in the road to release . Specifically, developer Team Folon has been having trouble getting it to work with Fallout 4’s newly released next-gen update.

Team Folon decided to delay Fallout: London in April, predicting that mods might encounter “complicating issues” when Bethesda’s next-gen update arrives – and the decision has proved wise . Last week, after some testing, the developer confirmed that Fallout 4’s next-gen update is “not stable enough” to run Fallout: London right now, and that the mod will be released with a “tool” rollback” to bring Fallout 4 back to an older version.

We already know that Fallout: London’s rollback tool will work with the GOG version of Fallout 4, since GOG is distributing the mod (its massive size makes it too big to host on Nexus Mods), but questions remain about versions of the game being purchased on other PC stores – which GOG has now answered.

In a statement to VG247, a spokesperson confirmed that Fallout: London will be compatible with the GOG version of Fallout 4. Additionally, it will work with the Steam release either through manual restocking or by using an upgrade rollback tool, which is currently “in progress.” However, copies of Fallout 4 distributed by Epic will not work with Fallout: London, as the Epic Store does not support the ability to rollback to previous versions of the game.

However, there is still hope that Epic owners will be able to participate in the final moments. Team Folon has confirmed that they are planning to update Fallout: London to work with Fallout 4’s next-gen patch as soon as Bethesda gets it into a more stable state.

The release date for Fallout: London on Steam and GOG is still uncertain. Team Folon recently mentioned that “the end is in sight,” but the mod must first pass GOG’s quality checks. They added, “Once they have finished retesting to ensure Fallout: London and its installer function on all supported machines, we will be ready.”