diff --git a/CHANGELOG.md b/CHANGELOG.md
index cf826dec4..1706b437f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 5.0.5
+
+* Fixed [#1667](https://github.com/mobxjs/mobx/issues/1667): creating a large array could result in undefined items (MobX 4.* was not affected)
+
 # 4.3.2 / 5.0.4
 
 * Fixed [#1685](https://github.com/mobxjs/mobx/issues/1685): expose `IAutorunOptions`
diff --git a/src/types/observablearray.ts b/src/types/observablearray.ts
index f65b685d8..68f523c6e 100644
--- a/src/types/observablearray.ts
+++ b/src/types/observablearray.ts
@@ -473,6 +473,7 @@ const arrayExtensions = {
  * faster as everything works on unproxied values
  */
 ;[
+    "concat",
     "every",
     "filter",
     "forEach",
diff --git a/test/base/array.js b/test/base/array.js
index 9fefbd8a8..fa49e8292 100644
--- a/test/base/array.js
+++ b/test/base/array.js
@@ -473,3 +473,22 @@ test("can define properties on arrays", () => {
     expect(ar.toString()).toBe("hoi")
     expect("" + ar).toBe("hoi")
 })
+
+test("concats correctly #1667", () => {
+    const x = observable({ data: [] })
+
+    function generate(count) {
+        const d = []
+        for (let i = 0; i < count; i++) d.push({})
+        return d
+    }
+
+    x.data = generate(10000)
+    const first = x.data[0]
+    expect(Array.isArray(x.data)).toBe(true)
+
+    x.data = x.data.concat(generate(1000))
+    expect(Array.isArray(x.data)).toBe(true)
+    expect(x.data[0]).toBe(first)
+    expect(x.data.length).toBe(11000)
+})