diff --git a/pkgs/development/python-modules/rsa/default.nix b/pkgs/development/python-modules/rsa/default.nix
index c062aad4c4b..1b3d339b078 100644
--- a/pkgs/development/python-modules/rsa/default.nix
+++ b/pkgs/development/python-modules/rsa/default.nix
@@ -4,6 +4,8 @@
 , unittest2
 , pyasn1
 , mock
+, isPy3k
+, pythonOlder
 }:
 
 buildPythonPackage rec {
@@ -18,6 +20,10 @@ buildPythonPackage rec {
   checkInputs = [ unittest2 mock ];
   propagatedBuildInputs = [ pyasn1 ];
 
+  preConfigure = stdenv.lib.optionalString (isPy3k && pythonOlder "3.7") ''
+    substituteInPlace setup.py --replace "open('README.md')" "open('README.md',encoding='utf-8')"
+  '';
+
   meta = with stdenv.lib; {
     homepage = https://stuvel.eu/rsa;
     license = licenses.asl20;