problem with array merg in SkeletonController::getSerializerContext

sorry for my english: My environment is Windows 7, Apache 2.4.6, PHP 5.5.4. Problem in '+':
$a = array('ss');
$b = array('sd');
$merged = $a + $b;
var_dump($merged); // array('ss')
////////////
$a = array('s'=>'ss');
$b = array('a'=>'sd');
$merged = $a + $b;
var_dump($merged); // array('s'=>'ss','a'=>'sd')

array_merge work properly