diff --git a/pkgs/development/tools/parsing/flex/2.5.39.nix b/pkgs/development/tools/parsing/flex/2.5.39.nix
index f12f89f6e24..93d54803e7c 100644
--- a/pkgs/development/tools/parsing/flex/2.5.39.nix
+++ b/pkgs/development/tools/parsing/flex/2.5.39.nix
@@ -12,7 +12,7 @@ stdenv.mkDerivation {
 
   propagatedNativeBuildInputs = [ m4 ];
 
-  postConfigure = stdenv.lib.optionalString stdenv.isDarwin ''
+  postConfigure = stdenv.lib.optionalString (stdenv.isDarwin || stdenv.isCygwin) ''
     sed -i Makefile -e 's/-no-undefined//;'
   '';