diff --git a/lifelines/fitters/coxph_fitter.py b/lifelines/fitters/coxph_fitter.py
index 2c23d433a..e26da10fc 100644
--- a/lifelines/fitters/coxph_fitter.py
+++ b/lifelines/fitters/coxph_fitter.py
@@ -282,7 +282,7 @@ def fit(self, df, duration_col, event_col=None,
         """
         df = df.copy()
         # Sort on time
-        df.sort(duration_col, inplace=True)
+        df.sort_values(by=duration_col, inplace=True)
 
         # remove strata coefs
         self.strata = strata