diff --git a/nixos/modules/security/acme/default.nix b/nixos/modules/security/acme/default.nix
index 5ffafdc37fef..83581d02840e 100644
--- a/nixos/modules/security/acme/default.nix
+++ b/nixos/modules/security/acme/default.nix
@@ -545,7 +545,7 @@ let
       };
 
       server = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         inherit (defaultAndText "server" "https://acme-v02.api.letsencrypt.org/directory") default defaultText;
         example = "https://acme-staging-v02.api.letsencrypt.org/directory";
         description = ''