diff --git a/docs/lib3x-zip.md b/docs/lib3x-zip.md
index 8f305f31..82953bf9 100644
--- a/docs/lib3x-zip.md
+++ b/docs/lib3x-zip.md
@@ -12,6 +12,7 @@ Make a list that aggregates elements from each of the lists. Returns a list of l
 ## Examples
 
     use <util/zip.scad>;
+    use <util/sum.scad>;
 
     xs = [10, 20, 30];
     ys = [5, 15, 25];