Invalid argument supplied for foreach()

I’ve written a simple plugin to add a couple of post types to my multisite installation. One of the post types has a meta field which contains a drop down list of posts from the other post type. However I’m getting the following error;

Invalid argument supplied for foreach() in blaa\wp-includes\query.php on line 1702

My code to populate the dropdown is as follows;

$properties = get_posts( array( 'post_type' => 'property', 'posts_per_page' => -1 ) );
$propertyList = array();

if( $properties ) {
	foreach( $properties as $property ) {
		$propertyList[] = array(
			'id' => $property->ID,
			'name' => $property->post_title
		);
	}
}

I’ve been looking and trying to fix this for a couple of hours now but I’m beaten. Has anyone any suggestions?