diff --git a/beta/src/pages/learn/updating-arrays-in-state.md b/beta/src/pages/learn/updating-arrays-in-state.md
index 90186f3e..c717a099 100644
--- a/beta/src/pages/learn/updating-arrays-in-state.md
+++ b/beta/src/pages/learn/updating-arrays-in-state.md
@@ -255,7 +255,9 @@ export default function ShapeEditor() {
         Move circles down!
       </button>
       {shapes.map(shape => (
-        <div style={{
+        <div
+          key={shape.id}
+          style={{
           background: 'purple',
           position: 'absolute',
           left: shape.x,