diff --git a/core/lib/Drupal/Core/Menu/LocalTaskDefault.php b/core/lib/Drupal/Core/Menu/LocalTaskDefault.php
index 44233e8..b6958f4 100644
--- a/core/lib/Drupal/Core/Menu/LocalTaskDefault.php
+++ b/core/lib/Drupal/Core/Menu/LocalTaskDefault.php
@@ -141,7 +141,7 @@ public function getTitle() {
   public function getWeight() {
     // By default the weight is 0, or -10 for the root tab.
     if (!isset($this->pluginDefinition['weight'])) {
-      if ($this->pluginDefinition['tab_root_id'] == $this->pluginDefinition['id']) {
+      if ($this->pluginDefinition['tab_root_id'] == $this->pluginId) {
         $this->pluginDefinition['weight'] = -10;
       }
       else {
diff --git a/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php b/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
index 6a7c8af..032a1c1 100644
--- a/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
+++ b/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
@@ -186,21 +186,34 @@ public function testGetRouteParametersForDynamicRouteWithUpcastedParameters() {
    */
   public function providerTestGetWeight() {
     return array(
-      array(array('weight' => 314), 314),
+      // Manually specify a weight, so this is used.
+      array(array('weight' => 314), 'test_id', 314),
       // Ensure that a default tab get a lower weight.
       array(
         array(
           'tab_root_id' => 'local_task_default',
           'id' => 'local_task_default'
         ),
+        'local_task_default',
         -10
       ),
+      // If the root ID is different to the ID of the tab, ignore it.
       array(
         array(
           'tab_root_id' => 'local_task_example',
           'id' => 'local_task_default'
         ),
-        0
+        'local_task_default',
+        0,
+      ),
+      // Ensure that a default tab of a derivative gets the default value.
+      array(
+        array(
+          'tab_root_id' => 'local_task_derivative_default:example_id',
+          'id' => 'local_task_derivative_default'
+        ),
+        'local_task_derivative_default:example_id',
+        -10,
       ),
     );
   }
@@ -212,8 +225,9 @@ public function providerTestGetWeight() {
    *
    * @see \Drupal\Core\Menu\LocalTaskDefault::getWeight()
    */
-  public function testGetWeight(array $plugin_definition, $expected_weight) {
+  public function testGetWeight(array $plugin_definition, $plugin_id, $expected_weight) {
     $this->pluginDefinition = $plugin_definition;
+    $this->pluginId = $plugin_id;
     $this->setupLocalTaskDefault();
 
     $this->assertEquals($expected_weight, $this->localTaskBase->getWeight());