diff --git a/extensions/mongodb/ActiveRecord.php b/extensions/mongodb/ActiveRecord.php
index 3fd3bad485..83fda1a125 100644
--- a/extensions/mongodb/ActiveRecord.php
+++ b/extensions/mongodb/ActiveRecord.php
@@ -222,7 +222,9 @@ abstract class ActiveRecord extends BaseActiveRecord
         if (empty($values)) {
             $currentAttributes = $this->getAttributes();
             foreach ($this->primaryKey() as $key) {
-                $values[$key] = isset($currentAttributes[$key]) ? $currentAttributes[$key] : null;
+                if (isset($currentAttributes[$key])) {
+                    $values[$key] = $currentAttributes[$key];
+                }
             }
         }
         $newId = static::getCollection()->insert($values);
diff --git a/extensions/mongodb/CHANGELOG.md b/extensions/mongodb/CHANGELOG.md
index 69936302cb..86c48461c7 100644
--- a/extensions/mongodb/CHANGELOG.md
+++ b/extensions/mongodb/CHANGELOG.md
@@ -4,6 +4,7 @@ Yii Framework 2 mongodb extension Change Log
 2.0.1 under development
 -----------------------
 
+- Bug #6026: Fixed `yii\mongodb\ActiveRecord` saves `null` as `_id`, if attributes are empty (klimov-paul)
 - Enh #3855: Added debug toolbar panel for MongoDB (klimov-paul)
 - Enh #5592: Added support for 'findAndModify' operation at `yii\mongodb\Query` and `yii\mongodb\ActiveQuery` (klimov-paul)
 
diff --git a/tests/unit/extensions/mongodb/ActiveRecordTest.php b/tests/unit/extensions/mongodb/ActiveRecordTest.php
index b89fdac4d9..177f5b17b2 100644
--- a/tests/unit/extensions/mongodb/ActiveRecordTest.php
+++ b/tests/unit/extensions/mongodb/ActiveRecordTest.php
@@ -275,4 +275,18 @@ class ActiveRecordTest extends MongoDbTestCase
         $this->assertTrue($customer instanceof Customer);
         $this->assertEquals($newName, $customer->name);
     }
+
+    /**
+     * @depends testInsert
+     *
+     * @see https://github.com/yiisoft/yii2/issues/6026
+     */
+    public function testInsertEmptyAttributes()
+    {
+        $record = new Customer();
+        $record->save(false);
+
+        $this->assertTrue($record->_id instanceof \MongoId);
+        $this->assertFalse($record->isNewRecord);
+    }
 }