diff --git a/python/prophet/forecaster.py b/python/prophet/forecaster.py
index 5905bf3..12caabf 100644
--- a/python/prophet/forecaster.py
+++ b/python/prophet/forecaster.py
@@ -320,7 +320,7 @@ class Prophet(object):
 
         if df.index.name == 'ds':
             df.index.name = None
-        df = df.sort_values('ds')
+        df = df.sort_values('ds', kind='mergesort')
         df = df.reset_index(drop=True)
 
         self.initialize_scales(initialize_scales, df)