diff --git a/nix/process-compose/settings/probe.nix b/nix/process-compose/settings/probe.nix
index 9fb7450..5f29d9d 100644
--- a/nix/process-compose/settings/probe.nix
+++ b/nix/process-compose/settings/probe.nix
@@ -79,7 +79,8 @@ in
           '';
         };
         options.working_dir = mkOption {
-          type = types.str;
+          type = types.nullOr types.str;
+          default = null;
           example = "./directory";
           description = ''
             Directory in which to execute the exec probe command.