diff --git a/pkgs/development/python-modules/cassandra-driver/default.nix b/pkgs/development/python-modules/cassandra-driver/default.nix
index bd577244aadf..d4a86bc566a7 100644
--- a/pkgs/development/python-modules/cassandra-driver/default.nix
+++ b/pkgs/development/python-modules/cassandra-driver/default.nix
@@ -66,7 +66,7 @@ buildPythonPackage rec {
   ++ lib.concatAttrValues optional-dependencies;
 
   # This is used to determine the version of cython that can be used
-  CASS_DRIVER_ALLOWED_CYTHON_VERSION = cython.version;
+  env.CASS_DRIVER_ALLOWED_CYTHON_VERSION = cython.version;
 
   preBuild = ''
     export CASS_DRIVER_BUILD_CONCURRENCY=$NIX_BUILD_CORES