diff --git a/src/ORM/ArrayList.php b/src/ORM/ArrayList.php
index c1231fb6b..0e42855d4 100644
--- a/src/ORM/ArrayList.php
+++ b/src/ORM/ArrayList.php
@@ -844,7 +844,7 @@ class ArrayList extends ViewableData implements SS_List, Filterable, Sortable, L
      */
     public function offsetSet($offset, $value)
     {
-        if ($offset == null) {
+        if ($offset === null) {
             $this->items[] = $value;
         } else {
             $this->items[$offset] = $value;
diff --git a/tests/php/ORM/ArrayListTest.php b/tests/php/ORM/ArrayListTest.php
index e28a03dfc..c6a586248 100644
--- a/tests/php/ORM/ArrayListTest.php
+++ b/tests/php/ORM/ArrayListTest.php
@@ -1251,4 +1251,14 @@ class ArrayListTest extends SapphireTest
 
         $this->assertNotEquals(range(1, $upperLimit), $list->toArray());
     }
+
+    public function testOffsetSet()
+    {
+        $list = new ArrayList(['first value', 'second value']);
+        $this->assertSame(2, $list->count());
+        $list->offsetSet(0, 'new value');
+        $this->assertSame(2, $list->count());
+        $this->assertSame('new value', $list->offsetGet(0));
+        $this->assertSame('second value', $list->offsetGet(1));
+    }
 }