diff --git a/tests/test_issue31_purge_residual.py b/tests/test_issue31_purge_residual.py
index bb21e4d..898d1c1 100644
--- a/tests/test_issue31_purge_residual.py
+++ b/tests/test_issue31_purge_residual.py
@@ -312,7 +312,9 @@ def test_any_spelling_of_the_uuid_is_the_lowercase_session(home, tmp_path, conv,
 
     live.kill()
     live.wait()
-    named = subprocess.Popen(["sh", "-c", "sleep 120", conv])
+    # Not `sh -c "sleep 120" <uuid>`: a shell execs its last simple command
+    # and the uuid would vanish from the process list (macOS /bin/sh).
+    named = subprocess.Popen([sys.executable, "-c", "import time; time.sleep(120)", conv])
     try:
         time.sleep(0.05)
         assert adapter.conversation_in_use(other, ctx) is True, "a command line naming the lowercase id"