diff --git a/src/EventManager.php b/src/EventManager.php
index f5d1597..aa8c12b 100644
--- a/src/EventManager.php
+++ b/src/EventManager.php
@@ -157,7 +157,7 @@ public function setIdentifiers($identifiers)
     public function addIdentifiers($identifiers)
     {
         if (is_array($identifiers) || $identifiers instanceof Traversable) {
-            $this->identifiers = array_unique($this->identifiers + (array) $identifiers);
+            $this->identifiers = array_unique(array_merge($this->identifiers, (array) $identifiers));
         } elseif ($identifiers !== null) {
             $this->identifiers = array_unique(array_merge($this->identifiers, array($identifiers)));
         }
diff --git a/test/EventManagerTest.php b/test/EventManagerTest.php
index 141a880..c4299ee 100644
--- a/test/EventManagerTest.php
+++ b/test/EventManagerTest.php
@@ -592,7 +592,12 @@ public function testIdentifierGetterSettersWorkWithArrays()
         $this->assertSame($this->events->getIdentifiers(), $identifiers);
         $identifiers[] = 'baz';
         $this->assertInstanceOf('Zend\EventManager\EventManager', $this->events->addIdentifiers($identifiers));
-        $this->assertSame($this->events->getIdentifiers(), $identifiers);
+
+        // This is done because the keys doesn't matter, just the values
+        $expectedIdentifiers = $this->events->getIdentifiers();
+        sort($expectedIdentifiers);
+        sort($identifiers);
+        $this->assertSame($expectedIdentifiers, $identifiers);
     }
 
     public function testIdentifierGetterSettersWorkWithTraversables()
@@ -602,7 +607,13 @@ public function testIdentifierGetterSettersWorkWithTraversables()
         $this->assertSame($this->events->getIdentifiers(), (array) $identifiers);
         $identifiers = new ArrayIterator(array('foo', 'bar', 'baz'));
         $this->assertInstanceOf('Zend\EventManager\EventManager', $this->events->addIdentifiers($identifiers));
-        $this->assertSame($this->events->getIdentifiers(), (array) $identifiers);
+
+        // This is done because the keys doesn't matter, just the values
+        $expectedIdentifiers = $this->events->getIdentifiers();
+        sort($expectedIdentifiers);
+        $identifiers = (array) $identifiers;
+        sort($identifiers);
+        $this->assertSame($expectedIdentifiers, $identifiers);
     }
 
     public function testListenersAttachedWithWildcardAreTriggeredForAllEvents()