diff --git a/core/lib/Drupal/Core/Menu/LocalTaskManager.php b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
index ff3f7de..013dff7 100644
--- a/core/lib/Drupal/Core/Menu/LocalTaskManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
@@ -147,7 +147,7 @@ protected function findDefinitions() {
       $found_routes[$definition['route_name']] = $definition['tab_root'];
     }
     $added_tabs = array();
-    foreach ($definitions as $plugin_id => $definition) {
+    foreach ($definitions as $plugin_id => &$definition) {
       if (!isset($found_routes[$definition['tab_root']])) {
         // Create the missing tab.
         $tabs = $this->createMissingTab($definition);
@@ -156,6 +156,12 @@ protected function findDefinitions() {
         }
         $added_tabs += $tabs;
       }
+      else {
+        // If a developer adds a new tab and references an existing tab as
+        // the root, we want to copy that existing tab's root so that the
+        // value is consistent.
+        $definition['tab_root'] = $found_routes[$definition['tab_root']];
+      }
     }
     return $definitions + $added_tabs;
   }
diff --git a/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php b/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
index 6a7c8af..1969937 100644
--- a/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
+++ b/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
@@ -190,15 +190,17 @@ public function providerTestGetWeight() {
       // Ensure that a default tab get a lower weight.
       array(
         array(
-          'tab_root_id' => 'local_task_default',
-          'id' => 'local_task_default'
+          'tab_root' => 'fake_route.default',
+          'route_name' => 'fake_route.default',
+          'id' => 'local_task_default',
         ),
         -10
       ),
       array(
         array(
-          'tab_root_id' => 'local_task_example',
-          'id' => 'local_task_default'
+          'tab_root' => 'fake_route.example',
+          'route_name' => 'fake_route.default',
+          'id' => 'local_task_default',
         ),
         0
       ),
diff --git a/core/tests/Drupal/Tests/Core/Menu/LocalTaskManagerTest.php b/core/tests/Drupal/Tests/Core/Menu/LocalTaskManagerTest.php
index f10bb38..641de10 100644
--- a/core/tests/Drupal/Tests/Core/Menu/LocalTaskManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Menu/LocalTaskManagerTest.php
@@ -269,20 +269,20 @@ protected function getLocalTaskFixtures() {
       'id' => 'menu_local_task_test_tasks_settings',
       'route_name' => 'menu_local_task_test_tasks_settings',
       'title' => 'Settings',
-      'tab_root_id' => 'menu_local_task_test_tasks_view',
+      'tab_root' => 'menu_local_task_test_tasks_view',
     );
     $definitions['menu_local_task_test_tasks_edit'] = array(
       'id' => 'menu_local_task_test_tasks_edit',
       'route_name' => 'menu_local_task_test_tasks_edit',
       'title' => 'Settings',
-      'tab_root_id' => 'menu_local_task_test_tasks_view',
+      'tab_root' => 'menu_local_task_test_tasks_view',
       'weight' => 20,
     );
     $definitions['menu_local_task_test_tasks_view'] = array(
       'id' => 'menu_local_task_test_tasks_view',
       'route_name' => 'menu_local_task_test_tasks_view',
       'title' => 'Settings',
-      'tab_root_id' => 'menu_local_task_test_tasks_view',
+      'tab_root' => 'menu_local_task_test_tasks_view',
     );
     // Add the defaults from the LocalTaskManager.
     foreach ($definitions as $id => &$info) {
@@ -291,7 +291,7 @@ protected function getLocalTaskFixtures() {
         'route_name' => '',
         'route_parameters' => array(),
         'title' => '',
-        'tab_root_id' => '',
+        'tab_root' => '',
         'tab_parent_id' => NULL,
         'weight' => 0,
         'options' => array(),