Can't run code on activation

Hello All,

I’ve scoured the net and read several tutorials that write the same code and I can’t figure out why my code, which is exactly the same, won’t run:

function WPMUSAS_plugin_install(){
	global $wpdb;

	$table_name = $wpdb->prefix . "wpmusas";

	if($wpdb->get_var("SHOW TABLES LIKE '$table_name'") != $table_name){
		$sql = "CREATE TABLE $table_name (
			ID int(5) NOT NULL AUTO_INCREMENT,
			country VARCHAR(255) DEFAULT '' NOT NULL,
			shipping VARCHAR(255) DEFAULT '' NOT NULL,
			UNIQUE KEY id (id)
		);";
		require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
    dbDelta($sql);
	}
}
register_activation_hook(__FILE__, 'WPMUSAS_install');

I’ve also tried trigger_error('TESTING', E_USER_ERROR); but even that just results in a “The plugin generated 238 characters of unexpected output during activation.” error message at the top.

What am I missing?