diff --git a/tests/test_optional_dependencies.py b/tests/test_optional_dependencies.py
index 77c0002..affdd05 100644
--- a/tests/test_optional_dependencies.py
+++ b/tests/test_optional_dependencies.py
@@ -22,7 +22,7 @@ sys.meta_path.insert(0, BlockSQLAlchemy())
 
 def _run_script(script: str) -> None:
     env = os.environ.copy()
-    env["PYTHONPATH"] = str(_PROJECT_ROOT / "src")
+    env["PYTHONPATH"] = env["PYTHONPATH"] + f":{_PROJECT_ROOT / 'src'}"
     result = subprocess.run(
         [sys.executable, "-c", script],
         cwd=_PROJECT_ROOT,