diff --git a/python/fbprophet/tests/test_prophet.py b/python/fbprophet/tests/test_prophet.py
index 25f23b6..fac9024 100644
--- a/python/fbprophet/tests/test_prophet.py
+++ b/python/fbprophet/tests/test_prophet.py
@@ -320,7 +320,7 @@ class TestProphet(TestCase):
             'upper_window': [1] * 2,
             'prior_scale': [8] * 2,
         })
-        holidays2 = pd.concat((holidays, holidays2))
+        holidays2 = pd.concat((holidays, holidays2), sort=True)
         m = Prophet(holidays=holidays2)
         feats, priors, names = m.make_holiday_features(df['ds'], m.holidays)
         pn = zip(priors, [s.split('_delim_')[0] for s in feats.columns])
@@ -332,7 +332,7 @@ class TestProphet(TestCase):
             'lower_window': [0] * 2,
             'upper_window': [1] * 2,
         })
-        holidays2 = pd.concat((holidays, holidays2))
+        holidays2 = pd.concat((holidays, holidays2), sort=True)
         feats, priors, names = Prophet(
             holidays=holidays2, holidays_prior_scale=4
         ).make_holiday_features(df['ds'], holidays2)