MySQL Syntax error when inserting

MySQL Syntax error when inserting

I've got a syntax error in the following code, but I can't find it:
$tableSelect = $_POST["tableSelect"];
$companyName = $_POST["companyName"];
$telephone = $_POST["telephone"];
$fax = $_POST["fax"];
$email = $_POST["email"];
$address = $_POST["address"];
$postcode = $_POST["postcode"];
$category = $_POST["category"];
$contact = $_POST["contact"];
$contactTel = $_POST["contactTel"];
$contactEmail = $_POST["contactEmail"];
$sql = "INSERT INTO '" . $tableSelect . "' ('" . $companyName . "', '" .
$telephone . "', '"
. $fax . "', '" . $email . "', '" . $address . "','" . $postcode . "',
'" . $category . "',
'" . $contact . "', '" . $contactTel . "', '" . $contactEmail . "')";
mysqli_query($con,$sql);
if (!mysqli_query($con,$sql)) {
die('Error: ' . mysqli_error($con));
}
Cheers!