diff --git a/models/behavior_analysis.py b/models/behavior_analysis.py
index 021e07b..edf1e33 100644
--- a/models/behavior_analysis.py
+++ b/models/behavior_analysis.py
@@ -33,7 +33,7 @@ class CombinationEvent:
         result = pd.Series(self.data[int(factors[0])]['values'][0])
         for i, opt in enumerate(opts):
             b = pd.Series(self.data[int(factors[i + 1])]['values'][0])
-            result = settings.ARITHMETIC[opt](result, b)
+            result = settings.ARITHMETIC[opt](result, b).fillna(0)
         if self.format == 'percent':
             result = round(result * 100, 2)
         elif self.format == 'float':