ช่วยดูหน่อยคับ Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource

<?php

function fetch_users(){
	$result = mysql_query('SELECT `user_id` AS `id`, `user_username` AS `username` FROM `users`');

	$users = array();

	while (($row = mysql_fetch_assoc($result)) !== false){
		$users[] = $row;
	}

	return $users;

}

//profile infomation
function fetch_user_info($uid){
	$uid = (int)$uid;

	$sql = "SELECT
				`user_username` AS `username`,
				`user_firstname` AS `firstname`,
				`user_lastname` AS `lastname`,
				`user_email` AS `email`,
				`user_anout` AS `about`,
				`user_location` AS `location`,
				`user_gender` AS `gender`
			FROM `users`
			WHERE `user_id` = {$uid}";

	$result = mysql_query($sql);
	
	return mysql_fetch_assoc($result);
}

?>

<?php

function fetch_users(){
	$sql = "SELECT `user_id` AS `id`, `user_username` AS `username` FROM `users`";
	$result = mysql_query($sql) or die(mysql_error());

	$users = array();

	while (($row = mysql_fetch_assoc($result)) !== false){
		$users[] = $row;
	}

	return $users;

}

//profile infomation
function fetch_user_info($uid){
	$uid = (int)$uid;

	$sql = "SELECT
				`user_username` AS `username`,
				`user_firstname` AS `firstname`,
				`user_lastname` AS `lastname`,
				`user_email` AS `email`,
				`user_anout` AS `about`,
				`user_location` AS `location`,
				`user_gender` AS `gender`
			FROM `users`
			WHERE `user_id` = {$uid}";

	$result = mysql_query($sql) or die(mysql_error());
	
	return mysql_fetch_assoc($result);
}

?>