diff --git a/config.org b/config.org
index a238470..5cc7716 100644
--- a/config.org
+++ b/config.org
@@ -1230,7 +1230,7 @@ If IMPURE is t, then allow impure builds."
   (with-temp-message (format "Building \"%s\" ..." out)
     (with-temp-buffer
       (let* ((args `("build" "--no-link" "--print-out-paths"
-                     ,@(if impure "--impure") ,out))
+                     ,@(when impure '("--impure")) ,out))
              (status (apply #'call-process nix-executable nil
                             (list (current-buffer) nil) nil args)))
         (when (eql status 0)