diff --git a/pkgs/development/tools/parsing/flex/default.nix b/pkgs/development/tools/parsing/flex/default.nix
index 42b007300a9..0bc26db5750 100644
--- a/pkgs/development/tools/parsing/flex/default.nix
+++ b/pkgs/development/tools/parsing/flex/default.nix
@@ -34,8 +34,8 @@ stdenv.mkDerivation rec {
   propagatedBuildInputs = [ m4 ];
 
   preConfigure = lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
-    "ac_cv_func_malloc_0_nonnull=yes"
-    "ac_cv_func_realloc_0_nonnull=yes"
+    "export ac_cv_func_malloc_0_nonnull=yes"
+    "export ac_cv_func_realloc_0_nonnull=yes"
   ];
 
   postConfigure = lib.optionalString (stdenv.isDarwin || stdenv.isCygwin) ''