diff --git a/index.js b/index.js
index ab8ab9f..ca49e94 100644
--- a/index.js
+++ b/index.js
@@ -25,6 +25,10 @@ function mixin(target, val, key) {
   let obj = target[key];
   if (isObject(val) && isObject(obj)) {
     mixinDeep(obj, val);
+  } else if (!(key in target) && isObject(val)) {
+    // `target` can be mutated by later mixins. Therefore, if we are currently adding an object to `target`,
+    // it could be mutated later. We need to make sure that mutation will not change the original data.
+    target[key] = {...val};
   } else {
     target[key] = val;
   }
diff --git a/package.json b/package.json
index 48d470c..0386923 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "mixin-deep",
   "description": "Deeply mix the properties of objects into the first object. Like merge-deep, but doesn't clone. No dependencies.",
-  "version": "2.0.1",
+  "version": "3.0.0",
   "homepage": "https://github.com/jonschlinkert/mixin-deep",
   "author": "Jon Schlinkert (https://github.com/jonschlinkert)",
   "repository": "jonschlinkert/mixin-deep",
diff --git a/test.js b/test.js
index 396a580..92f18c0 100644
--- a/test.js
+++ b/test.js
@@ -36,6 +36,19 @@ describe('.mixinDeep()', function() {
     assert.notDeepEqual(actual, obj3);
   });
 
+  it('should copy properties onto the first object without modifying other sub-objects', function() {
+    const obj1 = { x: { a: 0, b: 1 } };
+    const obj2 = { x: { c: 2, d: 3 } };
+    const obj3 = { x: { a: 4, d: 5 } };
+
+    const actual = { x: { a: 4, b: 1, c: 2, d: 5 } };
+
+    assert.deepEqual(mixinDeep({}, obj1, obj2, obj3), actual);
+    assert.notDeepEqual(actual, obj1);
+    assert.notDeepEqual(actual, obj2);
+    assert.notDeepEqual(actual, obj3);
+  });
+
   it('should mixin nested object properties', function() {
     const obj1 = { a: { b: 1, c: 1, d: { e: 1, f: 1 } } };
     const obj2 = { a: { b: 2, d: { f: 'f' } } };
@@ -79,7 +92,7 @@ describe('.mixinDeep()', function() {
 
     const actual = mixinDeep({}, obj1, obj2);
     assert.deepEqual(actual, { a: { b: 1, c: 2 } });
-    assert.deepEqual(actual.a, obj1.a);
+    assert.notDeepEqual(actual.a, obj1.a);
     assert.notDeepEqual(actual.a, obj2.a);
   });