Mysqli query returns FALSE instead of returning database records

Question

Hello, I’m trying to query the user login data in the php database but I noticed that the variable that receives the function that makes the query always returns empty and I var_dump this variable that showed me only the FALSE value(Which indicates there was an error in the query, right?). Below the code for better understanding.

Error:

Warning: mysqli_fetch_assoc() expects Parameter 1 to be mysqli_result, Boolean Given in /opt/lampp/htdocs/projects/centralgames/dao/usuario_dao.php on line 35 NULL

'valida_login.php' is called by login form and receives user data via 'POST'.

<?php

session_start();

//Recebe os campos da página login
$input_usuario = $_POST['usuario'];
$input_senha = $_POST['senha'];

include ('../dao/usuario_dao.php');

$usuario = seleciona_usuario($input_usuario, $input_senha);

//Se o resultado retornar vazio executa
if(empty($usuario))
{
    //Mensagem de erro
    $_SESSION['login_erro'] = "Usuário e/ou senha inválido(s)! Por favor, tente novamente.";


    var_dump($usuario);
    //Redireciona para a tela de login
    //header('Location: ../index.php');
}
else
{
    //Sessions que recebem os valores dos campos do banco de dados
    $_SESSION['nome_usuario'] = $usuario['nome'];
    $_SESSION['acesso_usuario'] = $usuario['permissao'];
    $_SESSION['login_usuario'] = $usuario['login'];

    $_SESSION['permissao'];

    if($_SESSION['acesso_usuario'] == 1)
    {
        $_SESSION['permissao'] = "Master";
        header('Location: menu_master.php');
    }
    else//Implementar outras permissões
    {
        $_SESSION['permissao'] = "Usuário";
        header("Location: menu_usuario.php");
    }
}

'usuario_dao.php' is where the function that makes the query in the database is.

<?php    
include ('../models/conexao.php');

//Função que seleciona usuário
function seleciona_usuario($login, $senha)
{
    $conn = conectar();

    $query = "SELECT * FROM tb_usuario WHERE (login='$login' AND senha=MD5('$senha'))";

    $result = mysqli_query($conn, $query);

    return mysqli_fetch_assoc($result);
}

Thank you in advance to those who can help.