diff --git a/lib/xmet/thermo.py b/lib/xmet/thermo.py
index f91c39a..6d1026c 100644
--- a/lib/xmet/thermo.py
+++ b/lib/xmet/thermo.py
@@ -182,7 +182,7 @@ def follow_saturated_mixing_ratio(temp: float, pressure: float, step: float=1.0)
     temp and pressure level, returning a Series object depicting the data
     points in descending pressure order.
     """
-    series = dict()
+    series = Series()
 
     ratio = saturated_mixing_ratio(temp, pressure)