diff --git a/exercises/practice/hamming/.meta/hamming.ys b/exercises/practice/hamming/.meta/hamming.ys
index 1878c97..072150b 100644
--- a/exercises/practice/hamming/.meta/hamming.ys
+++ b/exercises/practice/hamming/.meta/hamming.ys
@@ -3,4 +3,4 @@
 defn distance(strand1 strand2):
   strand1.# == strand2.# ||:
     die: 'strands must be of equal length'
-  sum: map(ne strand1 strand2).map(N)
+  len: map(ne strand1 strand2).filter(a)