diff --git a/pkgs/development/python-modules/tinycss/default.nix b/pkgs/development/python-modules/tinycss/default.nix
index d1023487c715..5419f93a0010 100644
--- a/pkgs/development/python-modules/tinycss/default.nix
+++ b/pkgs/development/python-modules/tinycss/default.nix
@@ -36,7 +36,7 @@ buildPythonPackage rec {
   '';
 
   # Disable Cython tests
-  TINYCSS_SKIP_SPEEDUPS_TESTS = true;
+  env.TINYCSS_SKIP_SPEEDUPS_TESTS = true;
 
   pythonImportsCheck = [ "tinycss" ];