diff --git a/core/includes/common.inc b/core/includes/common.inc
index 8a8ea0e85bb540e0a421332f5d96efeae489be8b..711d8d3a898eae540bf8a1d49fb56c789be7711a 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -4683,32 +4683,25 @@ function drupal_render(&$elements) {
   // element have to be rendered there. If the internal #render_children
   // property is set, do not call the #theme function to prevent infinite
   // recursion.
-  $theme_is_implemented = FALSE;
   if (isset($elements['#theme']) && !isset($elements['#render_children'])) {
     $elements['#children'] = theme($elements['#theme'], $elements);
-    // If theme() returns FALSE this means that the hook in #theme was not found
-    // in the registry. This is common for theme suggestions.
-    $theme_is_implemented = ($elements['#children'] !== FALSE);
-  }
-  // #theme is either not set or does not exist in the registry.
-  if (!$theme_is_implemented) {
-    // If #theme is not implemented and the element has children, render them
-    // now. This is the same process as drupal_render_children() but is inlined
-    // for speed.
+  }
+  // If #theme was not set and the element has children, render them now.
+  // This is the same process as drupal_render_children() but is inlined
+  // for speed.
+  if ($elements['#children'] === '') {
     foreach ($children as $key) {
       $elements['#children'] .= drupal_render($elements[$key]);
     }
-
-    // If #theme is not implemented and the element has raw #markup as a
-    // fallback, prepend the content in #markup to #children. In this case
-    // #children will contain whatever is provided by #pre_render prepended to
-    // what is rendered recursively above. If #theme is implemented then it is
-    // the responsibility of that theme implementation to render #markup if
-    // required. Eventually #theme_wrappers will expect both #markup and
-    // #children to be a single string as #children.
-    if (isset($elements['#markup'])) {
-      $elements['#children'] = $elements['#markup'] . $elements['#children'];
-    }
+  }
+  // If #theme was not set, but the element has raw #markup, prepend the content
+  // in #markup to #children. #children may contain the rendered content
+  // supplied by #theme, or the rendered child elements, as processed above. If
+  // both #theme and #markup are set, then #theme is responsible for rendering
+  // the element. Eventually assigned #theme_wrappers will expect both the
+  // element's #markup and the rendered content of child elements in #children.
+  if (!isset($elements['#theme']) && isset($elements['#markup'])) {
+    $elements['#children'] = $elements['#markup'] . $elements['#children'];
   }
 
   // Add any JavaScript state information associated with the element.
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index df03ad669af54afb9a8abffb3fadefbe7a451f3c..45737c5ccbc69900e90d7eab80e4bc0f44528b8f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -932,9 +932,8 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  *   properties are mapped to variables expected by the theme hook
  *   implementations.
  *
- * @return string|false
- *   An HTML string representing the themed output or FALSE if the passed $hook
- *   is not implemented.
+ * @return
+ *   An HTML string representing the themed output.
  *
  * @see themeable
  * @see hook_theme()
@@ -983,10 +982,7 @@ function theme($hook, $variables = array()) {
       if (!isset($candidate)) {
         watchdog('theme', 'Theme hook %hook not found.', array('%hook' => $hook), WATCHDOG_WARNING);
       }
-      // There is no theme implementation for the hook passed. Return FALSE so
-      // the function calling theme() can differentiate between a hook that
-      // exists and renders an empty string and a hook that is not implemented.
-      return FALSE;
+      return '';
     }
   }
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
index fac11fa5e26d9a458112a8918d421663657adcc2..31e4f340325c731c11cfefc0e15447b7495e1111 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
@@ -83,59 +83,6 @@ function testDrupalRenderBasics() {
     }
   }
 
-  /**
-   * Tests fallback rendering behaviour when #theme is not implemented.
-   *
-   * If #theme is set and is an implemented theme hook then theme() is 100%
-   * responsible for rendering the array including children and #markup.
-   *
-   * If #theme is not set or is not found in the registry then drupal_render()
-   * should recursively render child attributes of the array and #markup.
-   *
-   * This dual rendering behaviour is only relevant to the internal processing
-   * of drupal_render() before #theme_wrappers are called, so not #prefix and
-   * #suffix for example.
-   */
-  function testDrupalRenderFallbackRender() {
-    // Theme suggestion is not implemented, #markup should be rendered.
-    $theme_suggestion_not_implemented_has_markup = array(
-      '#theme' => array('suggestionnotimplemented'),
-      '#markup' => 'foo',
-    );
-    $rendered = drupal_render($theme_suggestion_not_implemented_has_markup);
-    $this->assertIdentical($rendered, 'foo');
-
-    // Theme suggestion is not implemented, children should be rendered.
-    $theme_suggestion_not_implemented_has_children = array(
-      '#theme' => array('suggestionnotimplemented'),
-      'child' => array(
-        '#markup' => 'foo',
-      ),
-    );
-    $rendered = drupal_render($theme_suggestion_not_implemented_has_children);
-    $this->assertIdentical($rendered, 'foo');
-
-    // Theme suggestion is implemented but returns empty string, #markup should
-    // not be rendered.
-    $theme_implemented_is_empty_has_markup = array(
-      '#theme' => array('common_test_empty'),
-      '#markup' => 'foo',
-    );
-    $rendered = drupal_render($theme_implemented_is_empty_has_markup);
-    $this->assertIdentical($rendered, '');
-
-    // Theme suggestion is implemented but returns empty string, children should
-    // not be rendered.
-    $theme_implemented_is_empty_has_children = array(
-      '#theme' => array('common_test_empty'),
-      'child' => array(
-        '#markup' => 'foo',
-      ),
-    );
-    $rendered = drupal_render($theme_implemented_is_empty_has_children);
-    $this->assertIdentical($rendered, '');
-  }
-
   /**
    * Tests sorting by weight.
    */
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 072bfb0ea0e0dfe2baffb8b0f4215a4590ab97ce..32f9321c255cea3d461174c91849c7095cccba49 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -204,7 +204,7 @@ function testRegistryRebuild() {
     // throws an exception.
     $this->rebuildContainer();
     $this->container->get('module_handler')->loadAll();
-    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'b')), FALSE, 'The theme registry does not contain theme_test_foo, because the module is disabled.');
+    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'b')), '', 'The theme registry does not contain theme_test_foo, because the module is disabled.');
 
     module_enable(array('theme_test'), FALSE);
     // After enabling/disabling a module during a test, we need to rebuild the
diff --git a/core/modules/system/tests/modules/common_test/common_test.module b/core/modules/system/tests/modules/common_test/common_test.module
index 175cd5ac0c3528ef0f32f978238713bff37757f7..83d19e0309a92bbe7dbb627c6b2c474498ac7434 100644
--- a/core/modules/system/tests/modules/common_test/common_test.module
+++ b/core/modules/system/tests/modules/common_test/common_test.module
@@ -164,26 +164,9 @@ function common_test_theme() {
       'variables' => array('foo' => 'foo', 'bar' => 'bar'),
       'template' => 'common-test-foo',
     ),
-    'common_test_empty' => array(
-      'variables' => array('foo' => 'foo'),
-    ),
   );
 }
 
-/**
- * Provides a theme function for drupal_render().
- */
-function theme_common_test_foo($variables) {
-  return $variables['foo'] . $variables['bar'];
-}
-
-/**
- * Always returns an empty string.
- */
-function theme_common_test_empty($variables) {
-  return '';
-}
-
 /**
  * Implements hook_library_info_alter().
  */