diff --git a/pkgs/by-name/fl/flac/package.nix b/pkgs/by-name/fl/flac/package.nix
index d8a654779866..93e57cc8952d 100644
--- a/pkgs/by-name/fl/flac/package.nix
+++ b/pkgs/by-name/fl/flac/package.nix
@@ -38,11 +38,13 @@ stdenv.mkDerivation (finalAttrs: {
     "-DBUILD_SHARED_LIBS=ON"
   ];
 
-  CFLAGS = [
-    "-O3"
-    "-funroll-loops"
-  ];
-  CXXFLAGS = [ "-O3" ];
+  env = {
+    CFLAGS = toString [
+      "-O3"
+      "-funroll-loops"
+    ];
+    CXXFLAGS = toString [ "-O3" ];
+  };
 
   patches = [ ./package.patch ];
   doCheck = true;