diff --git a/core/modules/system/tests/modules/theme_test/theme_test.inc b/core/modules/system/tests/modules/theme_test/theme_test.inc
index 6cde683..33622ab 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.inc
+++ b/core/modules/system/tests/modules/theme_test/theme_test.inc
@@ -13,3 +13,20 @@ function theme_theme_test($variables) {
 function template_preprocess_theme_test(&$variables) {
   $variables['foo'] = 'template_preprocess_theme_test';
 }
+
+/**
+ * Implements hook_preprocess_HOOK() for theme_theme_test_preprocess().
+ *
+ * Despite not having a corresponding theme function for this suggestion, the
+ * specific preprocess function should still be used.
+ */
+function template_preprocess_theme_test_preprocess__suggestion(&$variables) {
+  $variables['foo'] = 'template_preprocess_theme_test_preprocess__suggestion';
+}
+
+/**
+ * Returns HTML for the 'theme_test_preprocess' theme hook used by tests.
+ */
+function theme_theme_test_preprocess($variables) {
+  return 'Theme hook implementor=theme_theme_test_preprocess(). Foo=' . $variables['foo'];
+}
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 4ed90d2..3c0f0f1 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -8,6 +8,9 @@ function theme_test_theme($existing, $type, $theme, $path) {
     'file' => 'theme_test.inc',
     'variables' => array('foo' => ''),
   );
+  $items['theme_test_preprocess'] = array(
+    'variables' => array('foo' => ''),
+  );
   $items['theme_test_template_test'] = array(
     'template' => 'theme_test.template_test',
   );
@@ -41,6 +44,13 @@ function theme_test_menu() {
     'theme callback' => '_theme_custom_theme',
     'type' => MENU_CALLBACK,
   );
+  $items['theme-test/preprocess-suggestion'] = array(
+    'title' => 'Preprocess suggestion',
+    'page callback' => '_theme_test_preprocess_suggestion',
+    'access callback' => TRUE,
+    'theme callback' => '_theme_custom_theme',
+    'type' => MENU_CALLBACK,
+  );
   $items['theme-test/alter'] = array(
     'title' => 'Suggestion',
     'page callback' => '_theme_test_alter',
@@ -139,6 +149,13 @@ function _theme_test_suggestion() {
 }
 
 /**
+ * Page callback, calls a theme hook suggestion.
+ */
+function _theme_test_preprocess_suggestion() {
+  return theme(array('theme_test_preprocess__suggestion', 'theme_test_preprocess'), array());
+}
+
+/**
  * Implements hook_preprocess_HOOK() for html.tpl.php.
  */
 function theme_test_preprocess_html(&$variables) {
diff --git a/core/modules/system/tests/theme.test b/core/modules/system/tests/theme.test
index d7c6fb2..20dd57b 100644
--- a/core/modules/system/tests/theme.test
+++ b/core/modules/system/tests/theme.test
@@ -69,6 +69,22 @@ class ThemeUnitTest extends WebTestBase {
   }
 
   /**
+   * Ensures suggestion preprocess functions run even for default implementations.
+   *
+   * The theme hook used by this test has its base preprocess function in a
+   * separate file, so this test also ensures that that file is correctly loaded
+   * when needed.
+   */
+  function testSuggestionPreprocessForDefaults() {
+    // Test with both an unprimed and primed theme registry.
+    drupal_theme_rebuild();
+    for ($i = 0; $i < 2; $i++) {
+      $this->drupalGet('theme-test/preprocess-suggestion');
+      $this->assertText('Theme hook implementor=theme_theme_test_preprocess(). Foo=template_preprocess_theme_test_preprocess__suggestion', 'Theme hook ran with data available from a preprocess function for the suggested hook.');
+    }
+  }
+
+  /**
    * Ensure page-front template suggestion is added when on front page.
    */
   function testFrontPageThemeSuggestion() {