[Plugin: User Role Editor] Add new capability in a plugin

I would edit the “quotes collection” plugin (or others) to add a new capability defined in URE.
So, first I added a new role “quote_collection”.
Then, in the plugin I edit the code as follow:
from
add_action('admin_menu', 'schedeais_admin_menu');

to

if (current_user_can('quote_collection')) {
	add_action('admin_menu', 'schedeais_admin_menu');
}

But, when I reload the wp-admin this error appears:
Call to undefined function wp_get_current_user() in /home/server/site/wp-includes/capabilities.php on line 1028

So, someone can help me to add a capability to a plugin?
tnx

http://wordpress.org/extend/plugins/user-role-editor/