diff --git a/eto/core.py b/eto/core.py
index 9cb0dbb..9e55339 100644
--- a/eto/core.py
+++ b/eto/core.py
@@ -26,7 +26,7 @@ def __init__(self, df=None, freq='D', z_msl=None, lat=None, lon=None, TZ_lon=Non
         if df is None:
             pass
         else:
-            self.param_est(df, freq, z_msl, lat, lon, TZ_lon, z_u, K_rs, a_s, b_s, alb)
+            param_est(self, df, freq, z_msl, lat, lon, TZ_lon, z_u, K_rs, a_s, b_s, alb)
 
         ### Add in the ETo methods and other functions
         self.param_est = param_est