diff --git a/tests/conftest.py b/tests/conftest.py
index e9fe7b5..6580f2f 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -23,11 +23,11 @@ class TestSource(Source):
     self._future.set_exception(exc)
 
 @pytest.fixture(scope="module")
-async def run_source(*, clear_cache=False):
-  if clear_cache:
-    _cache.clear()
+async def run_source():
+  async def __call__(conf, *, clear_cache=False):
+    if clear_cache:
+      _cache.clear()
 
-  async def __call__(conf):
     future = asyncio.Future()
     file = io.StringIO(conf)
     file.name = '<StringIO>'