Mysqli join query problems

Hi,

I have a problem with this query.
What I want is to get the Category Name based on Category ID, each in different tables.

This is what I made so far, but it does not work:

<?php
# Output last 5 used categories
$sql = "SELECT
	inquiries.category, categories.name, categories.id
FROM
	inquiries, categories
WHERE
	userid='$userid'
JOIN
	category
ON
	inquiries.category = categories.id
ORDER BY
	inquiries.createdate
DESC LIMIT 5";
								
$res = $mysqli->query($sql);

while($cat = $res->fetch_array()) {
								
	echo '<option value="'.$cat['categories.id'].'">'.$cat['categories.name'].'</option>';
								
}
?>

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Error:

<b>Fatal error</b>:  Call to a member function fetch_array() on a non-object in <b>/home/d/i/ftp_dinethdk/m/inquiry.php</b> on line <b>196</b><br />

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