diff -u b/core/includes/config.inc b/core/includes/config.inc
--- b/core/includes/config.inc
+++ b/core/includes/config.inc
@@ -292,7 +292,7 @@
   $config_entities = array();
   $info = entity_get_info();
   foreach ($info as $entity_type => $entity_info) {
-    if ($entity_info['module'] == $module && isset($entity_info['config_prefix'])) {
+    if ($entity_info['module'] == $module && is_subclass_of($entity_info['class'], 'Drupal\Core\Config\Entity\ConfigEntityBase')) {
       $config_entities[$entity_type] = $entity_info;
     }
   }
diff -u b/core/modules/config/config.admin.inc b/core/modules/config/config.admin.inc
--- b/core/modules/config/config.admin.inc
+++ b/core/modules/config/config.admin.inc
@@ -94,7 +94,10 @@
  * Form submission handler for config_admin_import_form().
  */
 function config_admin_import_form_submit($form, &$form_state) {
-  if (config_import()) {
+  if (!lock()->lockMayBeAvailable(CONFIG_IMPORT_LOCK)) {
+    drupal_set_message(t('Another request may be synchronizing configuration already.'));
+  }
+  else if (config_import()) {
     // Once a sync completes, we empty the staging directory. This prevents
     // changes from being accidentally overwritten by stray files getting
     // imported later.
@@ -106,10 +109,6 @@
     drupal_set_message(t('The configuration was imported successfully.'));
   }
   else {
-    // Another request may be synchronizing configuration already. Wait for it
-    // to complete before returning the error, so already synchronized changes
-    // do not appear again.
-    lock()->wait(CONFIG_IMPORT_LOCK);
     drupal_set_message(t('The import failed due to an error. Any errors have been logged.'), 'error');
   }
 }
diff -u b/core/modules/config/lib/Drupal/config/Tests/ConfigImportUITest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigImportUITest.php
--- b/core/modules/config/lib/Drupal/config/Tests/ConfigImportUITest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigImportUITest.php
@@ -109,7 +109,7 @@
     // Attempt to import configuration and verify that an error message appears.
     $this->drupalPost(NULL, array(), t('Import all'));
     $this->assertUrl('admin/config/development/sync');
-    $this->assertText(t('The import failed due to an error. Any errors have been logged.'));
+    $this->assertText(t('Another request may be synchronizing configuration already.'));
 
     // Release the lock, just to keep testing sane.
     lock()->release('config_import');