diff --git a/nixos/modules/installer/cd-dvd/iso-image.nix b/nixos/modules/installer/cd-dvd/iso-image.nix
index fcdee1bc0960..3921bb61b91c 100644
--- a/nixos/modules/installer/cd-dvd/iso-image.nix
+++ b/nixos/modules/installer/cd-dvd/iso-image.nix
@@ -483,6 +483,7 @@ in
         Compression settings to use for the squashfs nix store.
       '';
       example = "zstd -Xcompression-level 6";
+      type = types.str;
     };
 
     isoImage.edition = mkOption {