diff --git a/tests/test_skill_index_prompt_injection.py b/tests/test_skill_index_prompt_injection.py
index 865e727bb..710a00032 100644
--- a/tests/test_skill_index_prompt_injection.py
+++ b/tests/test_skill_index_prompt_injection.py
@@ -105,7 +105,7 @@ def _patch_prefs(monkeypatch, data_dir):
         "skills_enabled": True,
         "auto_approve_skills": True,
     }
-    sys.modules["routes.prefs_routes"] = fake_prefs
+    monkeypatch.setitem(sys.modules, "routes.prefs_routes", fake_prefs)
 
     # Bust the base-prompt cache so our test re-reads the skill index.
     from src import agent_loop