diff --git a/nixos/modules/services/continuous-integration/buildbot/master.nix b/nixos/modules/services/continuous-integration/buildbot/master.nix
index 512e09eb8041..f638cf4bae3b 100644
--- a/nixos/modules/services/continuous-integration/buildbot/master.nix
+++ b/nixos/modules/services/continuous-integration/buildbot/master.nix
@@ -66,7 +66,7 @@ in {
       };
 
       masterCfg = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         description = ''
           Optionally pass raw master.cfg file as string.
           Other options in this configuration will be ignored.