diff --git a/changelog.md b/changelog.md
index 29b038d..96dff46 100644
--- a/changelog.md
+++ b/changelog.md
@@ -1,8 +1,9 @@
 # [5.0.0](https://github.com/TehShrike/deepmerge/releases/tag/v5.0.0)
 
-- Breaking: Dropped ES5 support. [#161](https://github.com/TehShrike/deepmerge/issues/161)
+- Breaking: The shipped bundle now targets ES2015 instead of ES5.  If you target IE11, you'll need to change your build process to compile dependencies. [#161](https://github.com/TehShrike/deepmerge/issues/161)
 - Breaking: by default, only [plain objects](https://github.com/sindresorhus/is-plain-obj/#is-plain-obj-) will have their properties merged, with all other values being copied to the target.  [#152](https://github.com/TehShrike/deepmerge/issues/152)
 - Breaking: the `isMergeableObject` option is renamed to `isMergeable` [#168](https://github.com/TehShrike/deepmerge/pull/168)
+- Fixed: the options argument is no longer mutated (again) [#173](https://github.com/TehShrike/deepmerge/pull/173)
 
 # [4.2.2](https://github.com/TehShrike/deepmerge/releases/tag/v4.2.2)
 
diff --git a/index.js b/index.js
index bf65974..2600ec5 100644
--- a/index.js
+++ b/index.js
@@ -70,13 +70,15 @@ const mergeObject = (target, source, options) => {
 	return destination
 }
 
-const deepmerge = (target, source, options) => {
-	options = Object.assign({
+const deepmerge = (target, source, inputOptions) => {
+	const options = {
 		arrayMerge: defaultArrayMerge,
 		isMergeable: defaultIsMergeable,
-	}, options, {
-		cloneUnlessOtherwiseSpecified,
-	})
+		...inputOptions,
+		// cloneUnlessOtherwiseSpecified is added to `options` so that custom arrayMerge()
+		// implementations can use it. The caller may not replace it.
+		cloneUnlessOtherwiseSpecified: cloneUnlessOtherwiseSpecified
+	}
 
 	const sourceIsArray = Array.isArray(source)
 	const targetIsArray = Array.isArray(target)
diff --git a/test/merge.js b/test/merge.js
index 1a99b28..acf0fa6 100644
--- a/test/merge.js
+++ b/test/merge.js
@@ -676,3 +676,12 @@ test('Falsey properties should be mergeable', function(t) {
 	t.ok(customMergeWasCalled, 'custom merge function was called')
 	t.end()
 })
+
+test('should not mutate options', function(t) {
+	var options = {}
+
+	merge({}, {}, options)
+
+	t.deepEqual(options, {})
+	t.end()
+})