diff --git a/core/includes/common.inc b/core/includes/common.inc
index 4087a42..654d849 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -5164,23 +5164,23 @@ function drupal_render_page($page) {
  * array to be rendered independently and prevents them from being rendered
  * more than once on subsequent calls to drupal_render() (e.g., as part of a
  * larger array). If the same array or array element is passed more than once
- * to drupal_render(), it simply returns a NULL value.
+ * to drupal_render(), it simply returns an empty string.
  *
- * @param $elements
+ * @param array $elements
  *   The structured array describing the data to be rendered.
  *
- * @return
+ * @return string
  *   The rendered HTML.
  */
 function drupal_render(&$elements) {
   // Early-return nothing if user does not have access.
   if (empty($elements) || (isset($elements['#access']) && !$elements['#access'])) {
-    return;
+    return '';
   }
 
   // Do not print elements twice.
   if (!empty($elements['#printed'])) {
-    return;
+    return '';
   }
 
   // Try to fetch the element's markup from cache and return.
@@ -5214,7 +5214,7 @@ function drupal_render(&$elements) {
 
   // Allow #pre_render to abort rendering.
   if (!empty($elements['#printed'])) {
-    return;
+    return '';
   }
 
   // Get the children of the element, sorted by weight.
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 8e053f9..31e4f34 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
@@ -30,6 +30,60 @@ public static function getInfo() {
   }
 
   /**
+   * Tests the output drupal_render() for some elementary input values.
+   */
+  function testDrupalRenderBasics() {
+    $types = array(
+      array(
+        'name' => 'null',
+        'value' => NULL,
+        'expected' => '',
+      ),
+      array(
+        'name' => 'no value',
+        'expected' => '',
+      ),
+      array(
+        'name' => 'empty string',
+        'value' => '',
+        'expected' => '',
+      ),
+      array(
+        'name' => 'no access',
+        'value' => array(
+          '#markup' => 'foo',
+          '#access' => FALSE,
+        ),
+        'expected' => '',
+      ),
+      array(
+        'name' => 'previously printed',
+        'value' => array(
+          '#markup' => 'foo',
+          '#printed' => TRUE,
+        ),
+        'expected' => '',
+      ),
+      array(
+        'name' => 'printed in prerender',
+        'value' => array(
+          '#markup' => 'foo',
+          '#pre_render' => array('common_test_drupal_render_printing_pre_render'),
+        ),
+        'expected' => '',
+      ),
+      array(
+        'name' => 'basic renderable array',
+        'value' => array('#markup' => 'foo'),
+        'expected' => 'foo',
+      ),
+    );
+    foreach($types as $type) {
+      $this->assertIdentical(drupal_render($type['value']), $type['expected'], '"' . $type['name'] . '" input rendered correctly by drupal_render().');
+    }
+  }
+
+  /**
    * Tests sorting by weight.
    */
   function testDrupalRenderSorting() {
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 203ead2..3670d41 100644
--- a/core/modules/system/tests/modules/common_test/common_test.module
+++ b/core/modules/system/tests/modules/common_test/common_test.module
@@ -148,6 +148,14 @@ function common_test_drupal_render_invalid_keys() {
 }
 
 /**
+ * Applies #printed to an element to help test #pre_render.
+ */
+function common_test_drupal_render_printing_pre_render($elements) {
+  $elements['#printed'] = TRUE;
+  return $elements;
+}
+
+/**
  * Implements hook_TYPE_alter().
  */
 function common_test_drupal_alter_alter(&$data, &$arg2 = NULL, &$arg3 = NULL) {