diff --git a/core/lib/Drupal/Core/Menu/StaticMenuLinkOverrides.php b/core/lib/Drupal/Core/Menu/StaticMenuLinkOverrides.php
index ebde906..fbde872 100644
--- a/core/lib/Drupal/Core/Menu/StaticMenuLinkOverrides.php
+++ b/core/lib/Drupal/Core/Menu/StaticMenuLinkOverrides.php
@@ -134,10 +134,9 @@ public function saveOverride($id, array $definition) {
     // Filter the overrides to only those that are expected.
     $definition = array_intersect_key($definition, $expected);
     if ($definition) {
-      $id = static::encodeId($id);
       $all_overrides = $this->getConfig()->get('definitions');
       // Combine with any existing data.
-      $all_overrides[$id] = $definition + $this->loadOverride($id);
+      $all_overrides[static::encodeId($id)] = $definition + $this->loadOverride($id);
       $this->getConfig()->set('definitions', $all_overrides)->save();
     }
     return array_keys($definition);
diff --git a/core/tests/Drupal/Tests/Core/Menu/StaticMenuLinkOverridesTest.php b/core/tests/Drupal/Tests/Core/Menu/StaticMenuLinkOverridesTest.php
index c436d6f..e68682f 100644
--- a/core/tests/Drupal/Tests/Core/Menu/StaticMenuLinkOverridesTest.php
+++ b/core/tests/Drupal/Tests/Core/Menu/StaticMenuLinkOverridesTest.php
@@ -105,52 +105,113 @@ public function testSaveOverride() {
     $config = $this->getMockBuilder('Drupal\Core\Config\Config')
       ->disableOriginalConstructor()
       ->getMock();
+    $config_factory = $this->getMock('Drupal\Core\Config\ConfigFactoryInterface');
+    $config_factory->expects($this->once())
+      ->method('get')
+      ->will($this->returnValue($config));
+    $static_override = new StaticMenuLinkOverrides($config_factory);
+
+    $existing_defintions = [];
+    $new_definitions = ['test1' => ['parent' => 'test0']];
     $config->expects($this->at(0))
       ->method('get')
       ->with('definitions')
-      ->will($this->returnValue(array()));
+      ->willReturn($existing_defintions);
     $config->expects($this->at(1))
       ->method('get')
       ->with('definitions')
-      ->will($this->returnValue(array()));
-
-    $definition_save_1 = array('definitions' => array('test1' => array('parent' => 'test0')));
-    $definitions_save_2 = array(
-      'definitions' => array(
-        'test1' => array('parent' => 'test0'),
-        'test1__la___ma' => array('parent' => 'test1')
-      )
-    );
+      ->willReturn($existing_defintions);
     $config->expects($this->at(2))
       ->method('set')
-      ->with('definitions', $definition_save_1['definitions'])
-      ->will($this->returnSelf());
+      ->with('definitions', $new_definitions)
+      ->willReturnSelf();
     $config->expects($this->at(3))
       ->method('save');
-    $config->expects($this->at(4))
-      ->method('get')
-      ->with('definitions')
-      ->will($this->returnValue($definition_save_1['definitions']));
-    $config->expects($this->at(5))
-      ->method('get')
-      ->with('definitions')
-      ->will($this->returnValue($definition_save_1['definitions']));
-    $config->expects($this->at(6))
-      ->method('set')
-      ->with('definitions', $definitions_save_2['definitions'])
-      ->will($this->returnSelf());
-    $config->expects($this->at(7))
-      ->method('save');
 
+    $static_override->saveOverride('test1', array('parent' => 'test0'));
+  }
+
+  /**
+   * Tests the saveOverride method when there are exisitng definitions.
+   *
+   * @covers ::saveOverride
+   * @covers ::loadOverride
+   * @covers ::getConfig
+   */
+  public function testSaveOverrideExistingDefinitions() {
+    $config = $this->getMockBuilder('Drupal\Core\Config\Config')
+                   ->disableOriginalConstructor()
+                   ->getMock();
+    $config_factory = $this->getMock('Drupal\Core\Config\ConfigFactoryInterface');
+    $config_factory->expects($this->once())
+                   ->method('get')
+                   ->will($this->returnValue($config));
+    $static_override = new StaticMenuLinkOverrides($config_factory);
+
+    $existing_definitions = ['test1' => ['parent' => 'test0']];
+    $new_definitions = [
+      'test1' => ['parent' => 'test0'],
+      'test1__la___ma' => ['parent' => 'test1']
+    ];
+    $config->expects($this->at(0))
+           ->method('get')
+           ->with('definitions')
+           ->willReturn($existing_definitions);
+    $config->expects($this->at(1))
+           ->method('get')
+           ->with('definitions')
+           ->willReturn($existing_definitions);
+    $config->expects($this->at(2))
+           ->method('set')
+           ->with('definitions', $new_definitions)
+           ->willReturnSelf();
+    $config->expects($this->at(3))
+           ->method('save');
+
+    $static_override->saveOverride('test1.la__ma', array('parent' => 'test1'));
+  }
+
+  /**
+   * Tests the saveOverride method when it has to merge configuration.
+   *
+   * @covers ::saveOverride
+   * @covers ::loadOverride
+   * @covers ::getConfig
+   */
+  public function testSaveOverrideMergeWithExistingDefinitions() {
+    $config = $this->getMockBuilder('Drupal\Core\Config\Config')
+      ->disableOriginalConstructor()
+      ->getMock();
     $config_factory = $this->getMock('Drupal\Core\Config\ConfigFactoryInterface');
     $config_factory->expects($this->once())
       ->method('get')
       ->will($this->returnValue($config));
-
     $static_override = new StaticMenuLinkOverrides($config_factory);
 
-    $static_override->saveOverride('test1', array('parent' => 'test0'));
-    $static_override->saveOverride('test1.la__ma', array('parent' => 'test1'));
+    $existing_definitions = [
+      'test1' => ['parent' => 'test0'],
+      'test1__la___ma' => ['parent' => 'test1']
+    ];
+    $new_definitions = [
+      'test1' => ['parent' => 'test0'],
+      'test1__la___ma' => ['parent' => 'test1', 'expanded'=> TRUE]
+    ];
+    $config->expects($this->at(0))
+      ->method('get')
+      ->with('definitions')
+      ->willReturn($existing_definitions);
+    $config->expects($this->at(1))
+      ->method('get')
+      ->with('definitions')
+      ->willReturn($existing_definitions);
+    $config->expects($this->at(2))
+      ->method('set')
+      ->with('definitions', $new_definitions)
+      ->willReturnSelf();
+    $config->expects($this->at(3))
+      ->method('save');
+
+    $static_override->saveOverride('test1.la__ma', array('expanded' => TRUE));
   }
 
   /**