diff --git a/pkgs/development/libraries/openwsman/default.nix b/pkgs/development/libraries/openwsman/default.nix
index 4df98d28fd09..a12abd5db557 100644
--- a/pkgs/development/libraries/openwsman/default.nix
+++ b/pkgs/development/libraries/openwsman/default.nix
@@ -23,7 +23,7 @@ stdenv.mkDerivation rec {
   ];
 
   preConfigure = ''
-    cmakeFlags="$cmakeFlags -DPACKAGE_ARCHITECTURE=$(uname -m)";
+    appendToVar cmakeFlags "-DPACKAGE_ARCHITECTURE=$(uname -m)"
   '';
 
   configureFlags = [ "--disable-more-warnings" ];