diff --git a/nixos/modules/virtualisation/virtualbox-image.nix b/nixos/modules/virtualisation/virtualbox-image.nix
index d31921f95ea..cd356b9d382 100644
--- a/nixos/modules/virtualisation/virtualbox-image.nix
+++ b/nixos/modules/virtualisation/virtualbox-image.nix
@@ -124,7 +124,7 @@ in {
       inherit pkgs lib config;
       partitionTableType = "legacy";
       diskSize = cfg.baseImageSize;
-      additionalSpace = cfg.baseImageFreeSpace;
+      additionalSpace = "${toString cfg.baseImageFreeSpace}M";
 
       postVM =
         ''