diff --git a/packages/effect-fc/src/Lens.ts b/packages/effect-fc/src/Lens.ts
index dc73344..f7f98c6 100644
--- a/packages/effect-fc/src/Lens.ts
+++ b/packages/effect-fc/src/Lens.ts
@@ -137,6 +137,23 @@ export const mapEffect: {
     )),
 }))
 
+export const mapStructAt: {
+    <A extends object, K extends keyof A, ER, EW, RR, RW>(
+        self: Lens<A, ER, EW, RR, RW>,
+        key: K,
+    ): Lens<A[K], ER, EW, RR, RW>
+    <A extends object, K extends keyof A, ER, EW, RR, RW>(
+        key: K,
+    ): (self: Lens<A, ER, EW, RR, RW>) => Lens<A[K], ER, EW, RR, RW>
+} = Function.dual(2, <A extends object, K extends keyof A, ER, EW, RR, RW>(
+    self: Lens<A, ER, EW, RR, RW>,
+    key: K,
+): Lens<A[K], ER, EW, RR, RW> => map(
+    self,
+    a => a[key],
+    (a, b) => Object.setPrototypeOf({ ...a, [key]: b }, Object.getPrototypeOf(a)),
+))
+
 export const mapArrayAt: {
     <A, ER, EW, RR, RW>(
         self: Lens<readonly A[], ER, EW, RR, RW>,
@@ -275,14 +292,15 @@ Effect.gen(function*() {
         values: [13, 69, 1488]
     })
 
-    const myValueLens = myChunkRef.pipe(
+    const myValueLens = ref.pipe(
         fromSubscriptionRef,
-        mapChunkAt(1),
+        mapStructAt("values"),
+        mapArrayAt(1),
     )
 
-    console.log(yield* myChunkRef.get, yield* chunkValueLens.get)
-    yield* set(chunkValueLens, 22)
-    console.log(yield* myChunkRef.get, yield* chunkValueLens.get)
+    console.log(yield* ref.get, yield* myValueLens.get)
+    yield* set(myValueLens, 22)
+    console.log(yield* ref.get, yield* myValueLens.get)
 }).pipe(
     Effect.runSync
 )