diff --git a/pkgs/by-name/gu/guacamole-server/package.nix b/pkgs/by-name/gu/guacamole-server/package.nix
index c9c7fefe3ae0..c79a68bdc5f7 100644
--- a/pkgs/by-name/gu/guacamole-server/package.nix
+++ b/pkgs/by-name/gu/guacamole-server/package.nix
@@ -37,7 +37,7 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-OjTwAQzKUuXfwZXLsL9XjrJc/0be38CmAGG+CoCeNwk=";
   };
 
-  NIX_CFLAGS_COMPILE = [
+  env.NIX_CFLAGS_COMPILE = toString [
     "-Wno-error=format-truncation"
     "-Wno-error=format-overflow"
     "-Wno-error=deprecated-declarations"