diff --git a/nixos/modules/services/cluster/k3s/default.nix b/nixos/modules/services/cluster/k3s/default.nix
index f0317fdbd160..e62fbc94415c 100644
--- a/nixos/modules/services/cluster/k3s/default.nix
+++ b/nixos/modules/services/cluster/k3s/default.nix
@@ -47,6 +47,7 @@ in
 
     extraFlags = mkOption {
       description = "Extra flags to pass to the k3s command.";
+      type = types.str;
       default = "";
       example = "--no-deploy traefik --cluster-cidr 10.24.0.0/16";
     };