diff --git a/yli/survival.py b/yli/survival.py
index 23457e5..e3456ff 100644
--- a/yli/survival.py
+++ b/yli/survival.py
@@ -331,7 +331,7 @@ def survtime_to_numeric(df, time, time2=None):
 		df[time] = df[time].dt.total_seconds()
 		max_time = df[time].max()
 	
-	if df[time2].dtype == '<m8[ns]':
+	if time2 and df[time2].dtype == '<m8[ns]':
 		df[time2] = df[time2].dt.total_seconds()
 		max_time = max(max_time or 0, df[time2].max())