diff --git a/pkgs/development/libraries/kde-frameworks/kimageformats.nix b/pkgs/development/libraries/kde-frameworks/kimageformats.nix
index 26a8637bafc..29748a5f7f4 100644
--- a/pkgs/development/libraries/kde-frameworks/kimageformats.nix
+++ b/pkgs/development/libraries/kde-frameworks/kimageformats.nix
@@ -12,5 +12,5 @@ mkDerivation {
   nativeBuildInputs = [ extra-cmake-modules ];
   buildInputs = [ karchive openexr qtbase ];
   outputs = [ "out" ]; # plugins only
-  NIX_CFLAGS_COMPILE = "-I${getDev ilmbase}/include/OpenEXR";
+  CXXFLAGS = "-I${getDev ilmbase}/include/OpenEXR";
 }