Fatal error: Call to undefined function wp_get_current_user

Hey all,

Got an odd error going on at the moment –

I have a plugin that uses the following code –

if(current_user_can('manage_options')){
	add_action('admin_menu', 'add_dd_options_menu');
	add_action('admin_head', 'dd_options_header');
}

But this plugin generates this error –

Fatal error: Call to undefined function wp_get_current_user() in C:\wamp\www\website\test\wp-includes\capabilities.php on line 1059

Changing it to the below so that I am not checking capabilities works fine –

//if(current_user_can('manage_options')){
	add_action('admin_menu', 'add_dd_options_menu');
	add_action('admin_head', 'dd_options_header');
//}

But the strangest thing is that this statement in a different file works fine, even when checking capabilities –

if(current_user_can('manage_options')){
	require_once('dd_includes/dashboard_widgets.php');
	add_action('admin_menu', 'remove_rubbish_dashboard_boxes');
}

Does anybody know if I am missing anything, or if i have made a very silly mistake with the code?

Any help apprciated.

Thanks.