diff --git a/packages/focal/src/utils.ts b/packages/focal/src/utils.ts
index 81f27ae..d8010b4 100644
--- a/packages/focal/src/utils.ts
+++ b/packages/focal/src/utils.ts
@@ -4,17 +4,16 @@ export { equals as structEq } from './equals'
 
 export const DEV_ENV = typeof process !== 'undefined' && process.env.NODE_ENV !== 'production'
 
-export function setKey<T, K extends keyof T>(k: K, v: T[K], o: T): T {
+export function setKey<T extends Object, K extends keyof T>(k: K, v: T[K], o: T): T {
   if (k in o && structEq(v, o[k])) {
     return o
   } else {
-    // this is the fastest way to do it, see
-    // https://jsperf.com/focal-setkey-for-loop-vs-object-assign
-    const r: { [k in keyof T]: T[k] } = {} as any
-    for (const p in o) r[p] = o[p]
-    r[k] = v
-
-    return r
+    // Faster to use Object.assign and friends in modern JS engines.
+    // Object.create needed to preserve the instance's prototype.
+    const copy: T = Object.create(o)
+    Object.assign(copy, o)
+    copy[k] = v
+    return copy
   }
 }
 
diff --git a/packages/focal/test/lens.test.ts b/packages/focal/test/lens.test.ts
index 1a9d71f..45b1404 100644
--- a/packages/focal/test/lens.test.ts
+++ b/packages/focal/test/lens.test.ts
@@ -202,3 +202,26 @@ describe('property expressions', () => {
     process.env.NODE_ENV = originalNodeEnv
   })
 })
+describe('object', () => {
+  class TestObject {
+    constructor(public num: number) {
+    }
+    getNum(): number {
+      return this.num
+    }
+  }
+  let o1 = new TestObject(0)
+  const l1 = Lens.key<TestObject>()('num')
+
+  it('num lens', () => expect(l1.get(o1)).toEqual(0))
+  it('num property', () => expect(o1.num).toEqual(0))
+
+  describe('after lens set', () => {
+    beforeEach(() => {
+      o1 = l1.set(10, o1)
+    })
+    it('num lens', () => expect(l1.get(o1)).toEqual(10))
+    it('num property', () => expect(o1.num).toEqual(10))
+    it('getNum method', () => expect(o1.getNum()).toEqual(10))
+  })
+})