PHP - ¿Por qué me manda error en consulta inner join con variables de sesión?

<?php

 session_start();

 fetch_data()

{

$output = '';

$conn = mysqli_connect("localhost", "root", "", "MYDB");

$conn -> set_charset("utf8");

$sql = "SELECT * FROM texto_menos50 T INNER JOIN empresasregistradas E ON

T.pin = E.pin WHERE T.pin ='{$_SESSION['pin']}' ORDER BY pin ASC";

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

while($row = mysqli_fetch_array($result))  <-- El error de la linea 12

{

$output .= 'CONTENIDO DEL PDF';}return $output;

Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result,

  boolean given in C:\wamp64\www\project\myproject.php on line

  12

  TCPDF ERROR: Some data has already been output, can't send PDF file

$session = $_SESSION['pin'];

$sql = "SELECT * FROM texto_menos50 T INNER JOIN empresasregistradas E ON

T.pin = E.pin WHERE T.pin = $session ORDER BY pin ASC";

$seguimiento = $mysqli->query("SELECT SS.$campo_seguimiento AS campo_seguimiento FROM solicitud_seguimiento SS WHERE SS.token = '$token' ") or trigger_error($mysqli->error);

$sql = "SELECT * FROM texto_menos50 T INNER JOIN empresasregistradas E ON

T.pin = E.pin WHERE T.pin ='{$_SESSION['pin']}' ORDER BY T.pin ASC";