diff --git a/tests/end-to-end/hooks/data_source/test_btrfs.py b/tests/end-to-end/hooks/data_source/test_btrfs.py
index 8ac0f524..15d8ec97 100644
--- a/tests/end-to-end/hooks/data_source/test_btrfs.py
+++ b/tests/end-to-end/hooks/data_source/test_btrfs.py
@@ -46,7 +46,7 @@ def test_btrfs_create_and_list():
         # Run a create action to exercise Btrfs snapshotting and backup.
         subprocess.check_call(
             f'borgmatic -v 2 --config {config_path} create'.split(' '),
-            env=dict(os.environ, **{'BTRFS_TEST_SUBVOLUME_PATH': '/e2e/mnt/subvolume'}),
+            env=dict(os.environ, BTRFS_TEST_SUBVOLUME_PATH='/e2e/mnt/subvolume'),
         )
 
         # List the resulting archive and assert that the snapshotted files are there.