diff --git a/nixos/modules/services/networking/unbound.nix b/nixos/modules/services/networking/unbound.nix
index f6e9634909241..34ed4e6fa031f 100644
--- a/nixos/modules/services/networking/unbound.nix
+++ b/nixos/modules/services/networking/unbound.nix
@@ -62,6 +62,7 @@ in {
       };
 
       stateDir = mkOption {
+        type = types.str;
         default = "/var/lib/unbound";
         description = "Directory holding all state for unbound to run.";
       };