diff --git a/pkgs/development/tools/parsing/flex/2.6.1.nix b/pkgs/development/tools/parsing/flex/2.6.1.nix
index 954c2bbcb24..8115ca19d02 100644
--- a/pkgs/development/tools/parsing/flex/2.6.1.nix
+++ b/pkgs/development/tools/parsing/flex/2.6.1.nix
@@ -8,6 +8,12 @@ stdenv.mkDerivation rec {
     sha256 = "0fy14c35yz2m1n1m4f02by3501fn0cca37zn7jp8lpp4b3kgjhrw";
   };
 
+  postPatch = ''
+    patchShebangs tests
+  '' + stdenv.lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''
+    substituteInPlace Makefile.in --replace "tests" " ";
+  '';
+
   buildInputs = [ bison ];
 
   propagatedBuildInputs = [ m4 ];
@@ -21,11 +27,6 @@ stdenv.mkDerivation rec {
     sed -i Makefile -e 's/-no-undefined//;'
   '';
 
-  # disable tests which can't run on build machine
-  postPatch = stdenv.lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''
-    substituteInPlace Makefile.in --replace "tests" " ";
-  '';
-
   meta = {
     homepage = https://github.com/westes/flex;
     description = "A fast lexical analyser generator";
diff --git a/pkgs/development/tools/parsing/flex/default.nix b/pkgs/development/tools/parsing/flex/default.nix
index 1bff2076b9a..52f7156ee80 100644
--- a/pkgs/development/tools/parsing/flex/default.nix
+++ b/pkgs/development/tools/parsing/flex/default.nix
@@ -19,7 +19,10 @@ stdenv.mkDerivation rec {
         + "/tools/flex/patches/200-build-AC_USE_SYSTEM_EXTENSIONS-in-configure.ac.patch";
     sha256 = "1aarhcmz7mfrgh15pkj6f7ikxa2m0mllw1i1vscsf1kw5d05lw6f";
   })];
-  postPatch = stdenv.lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) ''
+
+  postPatch = ''
+    patchShebangs tests
+  '' + stdenv.lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) ''
     substituteInPlace Makefile.in --replace "tests" " "
 
     substituteInPlace doc/Makefile.am --replace 'flex.1: $(top_srcdir)/configure.ac' 'flex.1: '