diff --git a/memory.py b/memory.py
index b5989da..bac5b69 100644
--- a/memory.py
+++ b/memory.py
@@ -55,7 +55,8 @@ We define a similarity measure on sets which counts the number of elements
 they have in common
 """
 def scalar(a, b):
-    return len(a.intersection(b))/sqrt(len(a)*len(b))
+    sizeProduct = len(a)*len(b)
+    return len(a.intersection(b))/sqrt(sizeProduct) if sizeProduct > 0 else 0
 
 def find_best_quote(db, user_input):
     indexed_input = index(user_input)