diff --git a/scikits/crab/recommenders/knn/classes.py b/scikits/crab/recommenders/knn/classes.py
index bf96af3..7e4e86d 100644
--- a/scikits/crab/recommenders/knn/classes.py
+++ b/scikits/crab/recommenders/knn/classes.py
@@ -532,12 +532,15 @@ def estimate_preference(self, user_id, item_id, **params):
 
         prefs = np.array([self.model.preference_value(to_user_id, item_id)
                  for to_user_id in nearest_neighbors])
-
-        prefs = prefs[~np.isnan(prefs)]
-        similarities = similarities[~np.isnan(prefs)]
-
-        prefs_sim = np.sum(prefs[~np.isnan(similarities)] *
-                             similarities[~np.isnan(similarities)])
+        
+        temp_prefs = [~np.isnan(prefs)]
+        temp_similarities = [~np.isnan(similarities)]
+        noNaN_indices = np.logical_and(temp_prefs, temp_similarities)
+        
+        prefs_sim = np.sum(prefs[noNaN_indices[0] == True] *
+                             similarities[noNaN_indices[0] == True])
+                             
+        similarities = similarities[~np.isnan(similarities)]
         total_similarity = np.sum(similarities)
 
         #Throw out the estimate if it was based on no data points,