class Loader implements \Twig_LoaderInterface, \Twig_ExistsLoaderInterface
{
    ...

    public function getSource($name)  // <------ SHOULD BE getSourceContext
    {
        ...
    }

    ...

interface Twig_LoaderInterface
{
    ...

    /**
     * Returns the source context for a given template logical name.
     *
     * @param string $name The template logical name
     *
     * @return Twig_Source
     *
     * @throws Twig_Error_Loader When $name is not found
     */
    public function getSourceContext($name); // <------ NEEDS THIS NAME
    {
        ...
    }

    ...