diff --git a/tests/unit/session/test_parallel.py b/tests/unit/session/test_parallel.py
index e08ef1f0c..3fa2331ba 100644
--- a/tests/unit/session/test_parallel.py
+++ b/tests/unit/session/test_parallel.py
@@ -272,8 +272,7 @@ def invoke_tox_in_thread(thread_name, result_json):
             # needs to be process to have it's own stdout
             invoke_result[thread_name] = subprocess.check_output(
                 [sys.executable, "-m", "tox", "-p", "all", "--result-json", str(result_json)],
-                universal_newlines=True,
-            )
+            ).decode("utf-8")
         except subprocess.CalledProcessError as exception:
             invoke_result[thread_name] = exception