diff --git a/pkgs/development/libraries/opencolorio/default.nix b/pkgs/development/libraries/opencolorio/default.nix
index 5ffa44c14037..64c6ef1defee 100644
--- a/pkgs/development/libraries/opencolorio/default.nix
+++ b/pkgs/development/libraries/opencolorio/default.nix
@@ -68,7 +68,7 @@ stdenv.mkDerivation rec {
 
   # Gcc blindly tries to optimize all float operations instead of just marked ones.
   # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122304
-  CXXFLAGS = "-ffp-contract=on";
+  env.CXXFLAGS = "-ffp-contract=on";
   cmakeFlags = [
     "-DOCIO_INSTALL_EXT_PACKAGES=NONE"
     "-DOCIO_USE_SSE2NEON=OFF"