Custom REST-API json-output not as expected

[Nico van Veenendaal]

I've got a weird issue with the json-output of my Custom REST-API.

The json-output of a standard REST-request is like:

{"1":{"value"}}

 The json-output of my custom REST-request is like:

"{\"1\":{\"value\"}}"

 As you can see the json-output is wrapped in double quotes and the other double are escaped.

The _retrieve()-function of my custom REST-API is:

    public function _retrieve()
    {
        $productCollection = Mage::getModel('sales/order')
            ->getCollection()
            ->addAttributeToSelect('*')
            ->load();

        $helper = Mage::helper('core');
        return $helper->jsonEncode($productCollection);
     }

What i'am missing here?