diff --git a/pkgs/applications/editors/zile/default.nix b/pkgs/applications/editors/zile/default.nix
index b7d7099a3472..801c0d384445 100644
--- a/pkgs/applications/editors/zile/default.nix
+++ b/pkgs/applications/editors/zile/default.nix
@@ -39,7 +39,7 @@ stdenv.mkDerivation rec {
   # fiddle with the terminal.
   doCheck = false;
 
-  env.NIX_CFLAGS_COMPILE = lib.optionals stdenv.cc.isClang "-Wno-error=incompatible-function-pointer-types";
+  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=incompatible-function-pointer-types";
 
   # XXX: Work around cross-compilation-unfriendly `gl_FUNC_FSTATAT' macro.
   gl_cv_func_fstatat_zero_flag="yes";