diff --git a/lib/make-disk-image.nix b/lib/make-disk-image.nix
index 572daa2..6343664 100644
--- a/lib/make-disk-image.nix
+++ b/lib/make-disk-image.nix
@@ -64,7 +64,7 @@ let
     ${systemToInstall.config.system.build.nixos-install}/bin/nixos-install --system ${systemToInstall.config.system.build.toplevel} --keep-going --no-channel-copy -v --no-root-password --option binary-caches ""
     umount -Rv ${systemToInstall.config.disko.rootMountPoint}
   '';
-  QEMU_OPTS = lib.concatMapStringsSep " " (disk: "-drive file=${disk.name}.raw,if=virtio,cache=unsafe,werror=report") (lib.attrValues nixosConfig.config.disko.devices.disk);
+  QEMU_OPTS = lib.concatMapStringsSep " " (disk: "-drive file=${disk.name}.raw,if=virtio,cache=unsafe,werror=report,format=raw") (lib.attrValues nixosConfig.config.disko.devices.disk);
 in
 {
   pure = vmTools.runInLinuxVM (pkgs.runCommand name