diff --git a/diffxpy/testing/utils.py b/diffxpy/testing/utils.py
index 6763a23..369d399 100644
--- a/diffxpy/testing/utils.py
+++ b/diffxpy/testing/utils.py
@@ -111,7 +111,7 @@ def parse_grouping(data, sample_description, grouping):
 
 def split_x(data, grouping):
     grouping = np.asarray(grouping)
-    groups = np.unique(grouping)
+    groups = pd.Series(grouping).unique()
     x0 = data[np.where(grouping == groups[0])[0]]
     x1 = data[np.where(grouping == groups[1])[0]]
     return x0, x1