diff --git a/diffxpy/testing/tests.py b/diffxpy/testing/tests.py
index 582d3b1..a7f56a7 100644
--- a/diffxpy/testing/tests.py
+++ b/diffxpy/testing/tests.py
@@ -912,7 +912,7 @@ def two_sample(
         test: str = "t-test",
         gene_names: Union[np.ndarray, list] = None,
         sample_description: pd.DataFrame = None,
-        noise_model: str = None,
+        noise_model: str = "nb",
         size_factors: np.ndarray = None,
         batch_size: Union[None, int, Tuple[int, int]] = None,
         backend: str = "numpy",