diff --git a/lib/modules/partition.nix b/lib/modules/partition.nix
index e565e09b..b806c22b 100644
--- a/lib/modules/partition.nix
+++ b/lib/modules/partition.nix
@@ -195,6 +195,8 @@
 
         disks = mkOption {
           type = types.attrsOf (mkDiskType false);
+          description = "The additional disks to format.";
+          default = {};
         };
 
         script = mkOption {