diff --git a/src/features/cr_features_helper_methods.py b/src/features/cr_features_helper_methods.py
index cc3aa216..9e96c497 100644
--- a/src/features/cr_features_helper_methods.py
+++ b/src/features/cr_features_helper_methods.py
@@ -1,4 +1,5 @@
 import pandas as pd
+import numpy as np
 import math as m
 
 import sys
@@ -39,7 +40,7 @@ def extract_second_order_features(intraday_features, so_features_names, prefix="
 
         # numWindowsNonZero specialized for BVP and IBI sensors
         if "hrv_num_windows_non_nan" in so_features_names and prefix+"meanHr" in intraday_features.columns:
-            so_features[prefix+"SO_numWindowsNonNaN"] = intraday_features.groupby(groupby_cols)[prefix+"meanHr"].apply(lambda x: (x!=0).sum())
+            so_features[prefix+"SO_numWindowsNonNaN"] = intraday_features.groupby(groupby_cols)[prefix+"meanHr"].apply(lambda x: (~np.isnan(x)).sum())
             
         so_features.reset_index(inplace=True)