diff --git a/src/Symfony/Component/Yaml/Parser.php b/src/Symfony/Component/Yaml/Parser.php
index da51e4930334e..46968b5d8a2f7 100644
--- a/src/Symfony/Component/Yaml/Parser.php
+++ b/src/Symfony/Component/Yaml/Parser.php
@@ -161,11 +161,7 @@ public function parse($value, $exceptionOnInvalidType = false, $objectSupport =
                             throw new ParseException('YAML merge keys used with a scalar value instead of an array.', $this->getRealCurrentLineNb() + 1, $this->currentLine);
                         }
 
-                        foreach ($refValue as $key => $value) {
-                            if (!isset($data[$key])) {
-                                $data[$key] = $value;
-                            }
-                        }
+                        $data += $refValue; // array union
                     } else {
                         if (isset($values['value']) && $values['value'] !== '') {
                             $value = $values['value'];
@@ -187,20 +183,12 @@ public function parse($value, $exceptionOnInvalidType = false, $objectSupport =
                                     throw new ParseException('Merge items must be arrays.', $this->getRealCurrentLineNb() + 1, $parsedItem);
                                 }
 
-                                foreach ($parsedItem as $key => $value) {
-                                    if (!isset($data[$key])) {
-                                        $data[$key] = $value;
-                                    }
-                                }
+                                $data += $parsedItem; // array union
                             }
                         } else {
                             // If the value associated with the key is a single mapping node, each of its key/value pairs is inserted into the
                             // current mapping, unless the key already exists in it.
-                            foreach ($parsed as $key => $value) {
-                                if (!isset($data[$key])) {
-                                    $data[$key] = $value;
-                                }
-                            }
+                            $data += $parsed; // array union
                         }
                     }
                 } elseif (isset($values['value']) && preg_match('#^&(?P<ref>[^ ]+) *(?P<value>.*)#u', $values['value'], $matches)) {
diff --git a/src/Symfony/Component/Yaml/Tests/Fixtures/sfMergeKey.yml b/src/Symfony/Component/Yaml/Tests/Fixtures/sfMergeKey.yml
index 4b67d341008e7..59f612514170a 100644
--- a/src/Symfony/Component/Yaml/Tests/Fixtures/sfMergeKey.yml
+++ b/src/Symfony/Component/Yaml/Tests/Fixtures/sfMergeKey.yml
@@ -10,9 +10,11 @@ yaml: |
         a: Steve
         b: Clark
         c: Brian
+        e: notnull
     bar:
         a: before
         d: other
+        e: ~
         <<: *foo
         b: new
         x: Oren
@@ -46,13 +48,13 @@ yaml: |
             <<: *nestedref
 php: |
     array(
-        'foo' => array('a' => 'Steve', 'b' => 'Clark', 'c' => 'Brian'),
-        'bar' => array('a' => 'before', 'd' => 'other', 'b' => 'new', 'c' => array('foo' => 'bar', 'bar' => 'foo'), 'x' => 'Oren'),
+        'foo' => array('a' => 'Steve', 'b' => 'Clark', 'c' => 'Brian', 'e' => 'notnull'),
+        'bar' => array('a' => 'before', 'd' => 'other', 'e' => null, 'b' => 'new', 'c' => array('foo' => 'bar', 'bar' => 'foo'), 'x' => 'Oren'),
         'duplicate' => array('foo' => 'bar'),
         'foo2' => array('a' => 'Ballmer'),
         'ding' => array('fi', 'fei', 'fo', 'fam'),
-        'check' => array('a' => 'Steve', 'b' => 'Clark', 'c' => 'Brian', 'fi', 'fei', 'fo', 'fam', 'isit' => 'tested'),
-        'head' => array('a' => 'Steve', 'b' => 'Clark', 'c' => 'Brian', 'fi', 'fei', 'fo', 'fam'),
+        'check' => array('a' => 'Steve', 'b' => 'Clark', 'c' => 'Brian', 'e' => 'notnull', 'fi', 'fei', 'fo', 'fam', 'isit' => 'tested'),
+        'head' => array('a' => 'Steve', 'b' => 'Clark', 'c' => 'Brian', 'e' => 'notnull', 'fi', 'fei', 'fo', 'fam'),
         'taz' => array('a' => 'Steve', 'w' => array('p' => 1234)),
         'nested' => array('a' => 'Steve', 'w' => array('p' => 12345), 'd' => 'Doug', 'z' => array('p' => 12345))
     )