diff --git a/hydra-module.nix b/hydra-module.nix
index 2aa8f28f..21e19f78 100644
--- a/hydra-module.nix
+++ b/hydra-module.nix
@@ -230,7 +230,7 @@ in
           hydra_logo ${cfg.logo}
         ''}
         gc_roots_dir ${cfg.gcRootsDir}
-        use-substitutes ${if cfg.useSubstitutes then "1" else "0"}
+        use-substitutes = ${if cfg.useSubstitutes then "1" else "0"}
       '';
 
     environment.systemPackages = [ cfg.package ];