diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 0938b57..aa76705 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1263,7 +1263,7 @@ function theme_invoke($hook, &$args = array()) {
     foreach ($theme_keys as $theme_key) {
       $function = $theme_key . '_' . $hook;
       if (function_exists($function)) {
-        return call_user_func_array($function, $args);
+        call_user_func_array($function, $args);
       }
     }
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
index 089ab48..9614924 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
@@ -36,31 +36,162 @@ function setUp() {
 
   function testThemePrepare() {
     $this->drupalGet('theme-test/prepare');
-    $this->assertRaw('<p>moduleVariable1: Drupal\theme_test\ThemeTestController::prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleVariable2: Drupal\theme_test\ThemeTestController::prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleVariableSuggestion1: Drupal\theme_test\ThemeTestController::prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleVariableSuggestion2: Drupal\theme_test\ThemeTestController::prepare|theme_test_theme_prepare_theme_test_prepare__implemented|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|test_theme_theme_prepare_theme_test_prepare__implemented|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter|theme_test_theme_prepare_theme_test_prepare__implemented_alter</p>');
-    $this->assertRaw('<p>moduleVariableSuggestion3: Drupal\theme_test\ThemeTestController::prepare|theme_test_theme_prepare_theme_test_prepare__implemented__extended|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|test_theme_theme_prepare_theme_test_prepare__implemented__extended|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter|theme_test_theme_prepare_theme_test_prepare__implemented__extended_alter</p>');
-    $this->assertRaw('<p>moduleElement1: Drupal\theme_test\ThemeTestController::prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleElementSuggestion1: Drupal\theme_test\ThemeTestController::prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleElementSuggestion2: Drupal\theme_test\ThemeTestController::prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleElementSuggestion3: Drupal\theme_test\ThemeTestController::prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
-    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter|test_theme_theme_prepare|test_theme_theme_prepare_theme_test_prepare|theme_test_theme_prepare_alter|test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleVariable1: Drupal\theme_test\ThemeTestController::prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleVariable2: Drupal\theme_test\ThemeTestController::prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleVariableSuggestion1: Drupal\theme_test\ThemeTestController::prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleVariableSuggestion2: Drupal\theme_test\ThemeTestController::prepare<br>
+theme_test_theme_prepare_theme_test_prepare__implemented<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+test_theme_theme_prepare_theme_test_prepare__implemented<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter<br>
+theme_test_theme_prepare_theme_test_prepare__implemented_alter</p>');
+
+    $this->assertRaw('<p>moduleVariableSuggestion3: Drupal\theme_test\ThemeTestController::prepare<br>
+theme_test_theme_prepare_theme_test_prepare__implemented__extended<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+test_theme_theme_prepare_theme_test_prepare__implemented__extended<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter<br>
+theme_test_theme_prepare_theme_test_prepare__implemented__extended_alter</p>');
+
+    $this->assertRaw('<p>moduleElement1: Drupal\theme_test\ThemeTestController::prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleElementSuggestion1: Drupal\theme_test\ThemeTestController::prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleElementSuggestion2: Drupal\theme_test\ThemeTestController::prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleElementSuggestion3: Drupal\theme_test\ThemeTestController::prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
+
+    $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
+test_theme_theme_prepare<br>
+test_theme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_theme_theme_prepare_alter</p>');
   }
 
 }
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index d771310..e0b2632 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -209,7 +209,7 @@ function theme_test_theme_prepare(&$variables, $hook) {
  */
 function theme_test_theme_prepare_alter(&$variables, $hook) {
   if ($hook === 'theme_test_prepare') {
-    $variables['value'] .= '|' . __FUNCTION__;
+    $variables['value'] .= "<br>\n" . __FUNCTION__;
   }
 }
 
@@ -217,28 +217,28 @@ function theme_test_theme_prepare_alter(&$variables, $hook) {
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION().
  */
 function theme_test_theme_prepare_theme_test_prepare__implemented(&$variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION_alter().
  */
 function theme_test_theme_prepare_theme_test_prepare__implemented_alter(&$variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION().
  */
 function theme_test_theme_prepare_theme_test_prepare__implemented__extended(&$variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION_alter().
  */
 function theme_test_theme_prepare_theme_test_prepare__implemented__extended_alter(&$variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
@@ -253,7 +253,7 @@ function theme_test_theme_prepare_theme_test_prepare_element(&$variables) {
 }
 
 /**
- * Implements hook_theme_prepare_THEME_HOOK().
+ * Implements hook_theme_prepare_THEME_HOOK_alter().
  */
 function theme_test_theme_prepare_theme_test_prepare_element_alter(&$variables) {
   $variables['element']['moduleInjectedVariable3'] = array(
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index 3ed2b73..99eb007 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -75,7 +75,7 @@ function test_theme_theme_test_function_suggestions__module_override($variables)
  */
 function test_theme_theme_prepare(&$variables, $hook) {
   if ($hook === 'theme_test_prepare') {
-    $variables['value'] .= '|' . __FUNCTION__;
+    $variables['value'] .= "<br>\n" . __FUNCTION__;
   }
   elseif ($hook === 'theme_test_prepare_element') {
     $variables['element']['themeInjectedVariable1'] = array(
@@ -91,7 +91,7 @@ function test_theme_theme_prepare(&$variables, $hook) {
  */
 function test_theme_theme_prepare_alter(&$variables, $hook) {
   if ($hook === 'theme_test_prepare') {
-    $variables['value'] .= '|' . __FUNCTION__;
+    $variables['value'] .= "<br>\n" . __FUNCTION__;
   }
 }
 
@@ -99,40 +99,40 @@ function test_theme_theme_prepare_alter(&$variables, $hook) {
  * Implements hook_theme_prepare_THEME_HOOK().
  */
 function test_theme_theme_prepare_theme_test_prepare(&$variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK_alter().
  */
 function test_theme_theme_prepare_theme_test_prepare_alter($variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION().
  */
 function test_theme_theme_prepare_theme_test_prepare__implemented($variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION_alter().
  */
 function test_theme_theme_prepare_theme_test_prepare__implemented_alter($variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION().
  */
 function test_theme_theme_prepare_theme_test_prepare__implemented__extended($variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION_alter().
  */
 function test_theme_theme_prepare_theme_test_prepare__implemented__extended_alter($variables) {
-  $variables['value'] .= '|' . __FUNCTION__;
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
 }