diff --git a/pkgs/by-name/sy/symmetrica/package.nix b/pkgs/by-name/sy/symmetrica/package.nix
index 44abbae5d9ae..8ce216072e8b 100644
--- a/pkgs/by-name/sy/symmetrica/package.nix
+++ b/pkgs/by-name/sy/symmetrica/package.nix
@@ -29,7 +29,10 @@ stdenv.mkDerivation (finalAttrs: {
 
   # clang warning: passing arguments to '...' without a prototype is deprecated
   # in all versions of C and is not supported in C23.
-  CFLAGS = "-std=c99 -Wno-deprecated-non-prototype";
+  env.CFLAGS = toString [
+    "-std=c99"
+    "-Wno-deprecated-non-prototype"
+  ];
 
   enableParallelBuilding = true;