diff --git a/feeds.api.php b/feeds.api.php
index 14a8555..c930972 100644
--- a/feeds.api.php
+++ b/feeds.api.php
@@ -292,6 +292,17 @@ function hook_feeds_processor_targets_alter(&$targets, $entity_type, $bundle_nam
       'callback' => 'my_module_set_target2',
       'real_target' => 'my_node_field_two', // Specify real target field on node.
     );
+    $targets['my_node_field3'] = array(
+      'name' => t('My third custom node field'),
+      'description' => t('Description of what my third custom node field does.'),
+      'callback' => 'my_module_set_target3',
+
+      // Set optional_unique to TRUE and specify unique_callbacks to allow the
+      // target to be unique. Existing entities can be updated based on unique
+      // targets.
+      'optional_unique' => TRUE,
+      'unique_callbacks' => array('my_module_mapper_unique'),
+    );
   }
 }
 
@@ -369,5 +380,38 @@ function my_module_form_callback($mapping, $target, $form, $form_state) {
 }
 
 /**
+ * Example of the unique_callbacks specified in
+ * hook_feeds_processor_targets_alter().
+ *
+ * @param string $target
+ *   A string identifying the unique target on the entity.
+ * @param mixed $value
+ *   The unique value to be checked.
+ * @param string $entity_type
+ *   Entity type for the entity to be processed.
+ * @param string $bundle_name
+ *   Bundle name for the entity to be processed.
+ *
+ * @return int|string|null
+ *   Feeds processor existing entity ID.
+ *   Or NULL if no existing entity is found.
+ *
+ * @see hook_feeds_processor_targets_alter()
+ * @see FeedsProcessor::existingEntityId()
+ */
+function my_module_mapper_unique($target, $value, $entity_type, $bundle_name) {
+  // Example for if the target is a field.
+  $query = new EntityFieldQuery();
+  $result = $query->entityCondition('entity_type', $entity_type)
+    ->entityCondition('bundle', $bundle_name)
+    ->fieldCondition($target, 'value', $value, '=')
+    ->execute();
+  if (isset($result[$entity_type])) {
+    $ids = array_keys($result[$entity_type]);
+    return reset($ids);
+  }
+}
+
+/**
  * @}
  */
diff --git a/plugins/FeedsProcessor.inc b/plugins/FeedsProcessor.inc
index e73b1ee..ec8f454 100755
--- a/plugins/FeedsProcessor.inc
+++ b/plugins/FeedsProcessor.inc
@@ -766,7 +766,7 @@ abstract class FeedsProcessor extends FeedsPlugin {
           if (isset($targets[$target]['unique_callbacks'])) {
             $callbacks = $targets[$target]['unique_callbacks'];
             foreach ($callbacks as $callback) {
-              if (function_exists($callback) && $entity_id = $callback($target, $value, $this->entityType(), $this->bundle())) {
+              if (is_callable($callback) && $entity_id = call_user_func_array($callback, array($target, $value, $this->entityType(), $this->bundle()))) {
                 // Stop at the first unique ID returned by a callback.
                 break;
               }
diff --git a/tests/feeds_mapper_unique.test b/tests/feeds_mapper_unique.test
new file mode 100644
index 0000000..8afaac7
--- /dev/null
+++ b/tests/feeds_mapper_unique.test
@@ -0,0 +1,101 @@
+<?php
+
+/**
+ * @file
+ * Test case for taxonomy mapper mappers/taxonomy.inc.
+ */
+
+/**
+ * Class for testing Feeds <em>content</em> mapper.
+ */
+class FeedsMapperUniqueTestCase extends FeedsMapperTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'Mapper: Unique target callbacks',
+      'description' => 'Test unique target callbacks in mappers.',
+      'group' => 'Feeds',
+    );
+  }
+
+  /**
+   * Test mapping target "unique_callbacks".
+   */
+  public function test() {
+    // Set unique callback.
+    variable_set('feeds_test_unique_callbacks', array(
+      array('FeedsMapperUniqueTestCase', 'getExistingEntityId')
+    ));
+
+    // Create content type.
+    $typename = $this->createContentType(array(), array(
+      'alpha' => 'text',
+    ));
+
+    // Create two nodes. Put unique value into field field_alpha.
+    $node1 = $this->drupalCreateNode(array(
+      'type' => $typename,
+      'field_alpha' => array(
+        LANGUAGE_NONE => array(
+          0 => array(
+            'value' => 'Ut wisi',
+          ),
+        ),
+      ),
+    ));
+    $node2 = $this->drupalCreateNode(array(
+      'type' => $typename,
+      'field_alpha' => array(
+        LANGUAGE_NONE => array(
+          0 => array(
+            'value' => 'Lorem',
+          ),
+        ),
+      ),
+    ));
+
+    // Create and configure importer.
+    $this->createImporterConfiguration('Syndication', 'syndication');
+    $this->setPlugin('syndication', 'FeedsFileFetcher');
+    $this->setPlugin('syndication', 'FeedsCSVParser');
+    $this->setSettings('syndication', 'FeedsNodeProcessor', array('bundle' => $typename, 'update_existing' => 2));
+    $this->addMappings('syndication', array(
+      0 => array(
+        'source' => 'title',
+        'target' => 'title',
+      ),
+      1 => array(
+        'source' => 'alpha',
+        'target' => 'test_target',
+        'unique' => TRUE,
+        'textfield' => 'required',
+      ),
+    ));
+
+    // Import CSV file.
+    $this->importFile('syndication', $this->absolutePath() . '/tests/feeds/content.csv');
+    $this->assertText('Updated 2 nodes');
+
+    // Ensure the updated nodes have the expected title now.
+    node_load($node1->nid, NULL, TRUE);
+    $this->assertEqual('Ut wisi enim ad minim veniam', $node1->title, 'Node 1 has the expected title');
+    node_load($node2->nid, NULL, TRUE);
+    $this->assertEqual('Lorem ipsum', $node2->title, 'Node 2 has the expected title');
+  }
+
+  /**
+   * Callback for unique_callbacks for test_target mapper.
+   *
+   * @see feeds_tests_feeds_processor_targets_alter()
+   */
+  public static function getExistingEntityId($target, $value, $entity_type, $bundle_name) {
+    $query = new EntityFieldQuery();
+    $result = $query->entityCondition('entity_type', $entity_type)
+      ->entityCondition('bundle', $bundle_name)
+      ->fieldCondition('field_alpha', 'value', $value, '=')
+      ->execute();
+    if (isset($result[$entity_type])) {
+      $ids = array_keys($result[$entity_type]);
+      return reset($ids);
+    }
+  }
+}
diff --git a/tests/feeds_tests.module b/tests/feeds_tests.module
index 399708b..a038535 100644
--- a/tests/feeds_tests.module
+++ b/tests/feeds_tests.module
@@ -112,6 +112,13 @@ function feeds_tests_feeds_processor_targets_alter(&$targets, $entity_type, $bun
     'summary_callback' => 'feeds_tests_mapper_summary',
     'form_callback' => 'feeds_tests_mapper_form',
   );
+
+  // Set unique callbacks for FeedsMapperUniqueTestCase.
+  $callbacks = variable_get('feeds_test_unique_callbacks');
+  if (!empty($callbacks)) {
+    $targets['test_target']['optional_unique'] = TRUE;
+    $targets['test_target']['unique_callbacks'] = $callbacks;
+  }
 }
 
 /**