Storing an XML Response (Transient)?

Haven’t worked much with XML so I’m hitting a bit of a wall:

function getapi() 
{
    $api_response = wp_remote_get( "http://example.com/getXML" );
    $data         = wp_remote_retrieve_body( $api_response );
    $output       = new SimpleXMLElement ($data );

    return $output;
}

Get or set the Transient

function transient()
{ 
    $transient = get_transient( 'transient_value' );

    if ( ! $transient ) {
        $transient = getapi();
        set_transient( 'transient_value', $transient, 180 );
    }

    return $transient;
}

I can easily show the data, but calling it up from a stored transient results in this error being shown:

Node no longer exists in C:xampplitehtdocs...

Not sure what the extra step is that I need to perform in order to store the data correctly.

Many thanks!

Noel