diff --git a/ch02.jl b/ch02.jl
index 58c01dc..c6f4818 100644
--- a/ch02.jl
+++ b/ch02.jl
@@ -196,7 +196,7 @@ y
 
 s = 0
 for v in y
-    s += v
+    global s += v
 end
 s
 s / length(y)