diff --git a/__tests__/base.js b/__tests__/base.js
index 2f0fc776..3382ffa5 100644
--- a/__tests__/base.js
+++ b/__tests__/base.js
@@ -2345,6 +2345,39 @@ function testObjectTypes(produce) {
 		expect(next.y).toBe(2)
 		expect(state.x).toBe(0)
 	})
+
+	describe("#620", () => {
+		const customSymbol = Symbol("customSymbol")
+
+		class TestClass {
+			[immerable] = true;
+			[customSymbol] = 1
+		}
+
+		/* Create class instance */
+		let test0 = new TestClass()
+
+		/* First produce. This works */
+		let test1 = produce(test0, draft => {
+			expect(draft[customSymbol]).toBe(1)
+			draft[customSymbol] = 2
+		})
+		expect(test1).toEqual({
+			[immerable]: true,
+			[customSymbol]: 2
+		})
+
+		/* Second produce. This does NOT work. See console error */
+		/* With version 6.0.9, this works though */
+		let test2 = produce(test1, draft => {
+			expect(draft[customSymbol]).toBe(2)
+			draft[customSymbol] = 3
+		})
+		expect(test2).toEqual({
+			[immerable]: true,
+			[customSymbol]: 3
+		})
+	})
 }
 
 function testLiteralTypes(produce) {
diff --git a/src/plugins/es5.ts b/src/plugins/es5.ts
index a0ae53cd..65918b60 100644
--- a/src/plugins/es5.ts
+++ b/src/plugins/es5.ts
@@ -16,7 +16,8 @@ import {
 	getCurrentScope,
 	die,
 	markChanged,
-	objectTraps
+	objectTraps,
+	ownKeys
 } from "../internal"
 
 type ES5State = ES5ArrayState | ES5ObjectState
@@ -49,7 +50,9 @@ export function enableES5() {
 		// Descriptors we want to skip:
 		if (isArray) delete descriptors.length
 		delete descriptors[DRAFT_STATE as any]
-		for (let key in descriptors) {
+		const keys = ownKeys(descriptors)
+		for (let i = 0; i < keys.length; i++) {
+			const key: any = keys[i]
 			descriptors[key] = proxyProperty(
 				key,
 				isArray || !!descriptors[key].enumerable
@@ -204,9 +207,10 @@ export function enableES5() {
 
 		// Search for added keys and changed keys. Start at the back, because
 		// non-numeric keys are ordered by time of definition on the object.
-		const keys = Object.keys(draft_)
+		const keys = ownKeys(draft_)
 		for (let i = keys.length - 1; i >= 0; i--) {
-			const key = keys[i]
+			const key: any = keys[i]
+			if (key === DRAFT_STATE) continue
 			const baseValue = base_[key]
 			// The `undefined` check is a fast path for pre-existing keys.
 			if (baseValue === undefined && !has(base_, key)) {
@@ -225,7 +229,8 @@ export function enableES5() {
 
 		// At this point, no keys were added or changed.
 		// Compare key count to determine if keys were deleted.
-		return keys.length !== Object.keys(base_).length
+		const baseIsDraft = !!base_[DRAFT_STATE as any]
+		return keys.length !== ownKeys(base_).length + (baseIsDraft ? 0 : 1) // + 1 to correct for DRAFT_STATE
 	}
 
 	function hasArrayChanges(state: ES5ArrayState) {
diff --git a/src/utils/common.ts b/src/utils/common.ts
index 43545d0f..c78121f8 100644
--- a/src/utils/common.ts
+++ b/src/utils/common.ts
@@ -146,7 +146,9 @@ export function shallowCopy(base: any) {
 	if (Array.isArray(base)) return base.slice()
 	const descriptors = Object.getOwnPropertyDescriptors(base)
 	delete descriptors[DRAFT_STATE as any]
-	for (let key in descriptors) {
+	let keys = ownKeys(descriptors)
+	for (let i = 0; i < keys.length; i++) {
+		const key: any = keys[i]
 		const desc = descriptors[key]
 		if (desc.writable === false) {
 			desc.writable = true