diff --git a/test/util.py b/test/util.py
index 696db87e4..b01a69c30 100644
--- a/test/util.py
+++ b/test/util.py
@@ -98,10 +98,12 @@ def arg_fun(x):
     }
 
 
-def rosen_for_sensi(max_sensi_order, integrated=False, x=None):
+def rosen_for_sensi(
+    max_sensi_order, integrated=False, x: np.ndarray | None = None
+):
     """Rosenbrock function from scipy.optimize."""
     if x is None:
-        x = [0, 1]
+        x = np.array([0, 1])
 
     return obj_for_sensi(
         so.rosen, so.rosen_der, so.rosen_hess, max_sensi_order, integrated, x