diff --git a/pkgs/development/libraries/libedit/default.nix b/pkgs/development/libraries/libedit/default.nix
index f0eb267b05b57..d8c7be41f4a10 100644
--- a/pkgs/development/libraries/libedit/default.nix
+++ b/pkgs/development/libraries/libedit/default.nix
@@ -21,9 +21,9 @@ stdenv.mkDerivation rec {
   # There is a DR to fix this issue with Clang which is not merged
   # yet.
   # https://reviews.llvm.org/D137043
-  NIX_CFLAGS_COMPILE = lib.optional
-    (stdenv.targetPlatform.isMusl && stdenv.cc.isClang)
-    "-D__STDC_ISO_10646__=201103L";
+  env.NIX_CFLAGS_COMPILE =
+    lib.optionalString (stdenv.targetPlatform.isMusl && stdenv.cc.isClang)
+      "-D__STDC_ISO_10646__=201103L";
 
   patches = [ ./01-cygwin.patch ];
 
diff --git a/pkgs/tools/archivers/sharutils/default.nix b/pkgs/tools/archivers/sharutils/default.nix
index 002854e045d22..5c84e596531c3 100644
--- a/pkgs/tools/archivers/sharutils/default.nix
+++ b/pkgs/tools/archivers/sharutils/default.nix
@@ -59,7 +59,7 @@ stdenv.mkDerivation rec {
     '';
 
   # Workaround to fix the static build on macOS.
-  NIX_CFLAGS_COMPILE = "-Wno-implicit-function-declaration";
+  env.NIX_CFLAGS_COMPILE = "-Wno-implicit-function-declaration";
 
   doCheck = true;