is_active_sidebar() always returns false

I’m inserting a widget area (on a hook) on a page like so:

function themeblvd_custom_header_addon_content() {
	if(is_active_sidebar('header-sidebar')){ ?>
	<div class="jma-header-right">
		<?php dynamic_sidebar('header-sidebar')   ?>
	</div>
	<?php }
}
add_action( 'themeblvd_header_addon', 'themeblvd_custom_header_addon_content' );

registering the widget area like so:

register_sidebar(array(
            'name' => 'header sidebar',
            'id' => 'header-sidebar',
            'description' => 'This is the widget area for the header',
            'before_widget' => '<div class="widget %2$s" id="%1$s">',
            'after_widget' => '</div>',
            'before_title' => '<h3>',
            'after_title' => '</h3>'
            ));

When the widget area contains a widget I still get no display. The widget does display if I remove the condition.
Am I mis-understanding this function. I expect that when a widget is “dragged” into the widget area the widget area is “active” and this function should return TRUE.