diff --git a/spiderexpress/strategies/spikyball.py b/spiderexpress/strategies/spikyball.py
index 2727b44..e61d722 100644
--- a/spiderexpress/strategies/spikyball.py
+++ b/spiderexpress/strategies/spikyball.py
@@ -96,7 +96,10 @@ def calc_norm(source: pd.Series, edge: pd.Series, target: pd.Series) -> float:
 
     float : the normalization constant
     """
-    return sum(source.fillna(1) * edge.fillna(1) * target.fillna(1))
+    if any(source.isna(), edge.isna(), target.isna()):
+        log.warning("Input contains NaN values which will be replaced with 1.")
+    norm_const = (source.fillna(1) * edge.fillna(1) * target.fillna(1)).fillna(1).sum()
+    return norm_const
 
 
 def calc_prob(table: pd.DataFrame, params: ProbabilityConfiguration) -> pd.Series: