mysqli_free_result - select successful, but still getting warning - mysql

Question

I have a problem whereby I am running a SELECT query that returns a valid result set. I process the results using mysqli_fetch_assoc and then pass the result set to mysqli_free_result, which returns false. I have done a var_dump of the result before and after the call to confirm it is a valid result (seems to be to me - I can access the data I expect to). The query I am running is something like:
select us.user_session_key
, u.user_key
, ut.code as user_type
from t_user u
inner join t_user_type ut on u.user_type_key = ut.user_type_key
inner join t_user_session us
on u.user_key = us.user_key
where u.email = 'tim.super#en.com'
and us.session_id = 'mjsvu6fru3dtfk1bbs59fc8t20'
and us.expired_yn = 'N'
order by 1 desc
Var dump of the result object:
object(mysqli_result)#2 (5) {
["current_field"]=>
int(0)
["field_count"]=>
int(3)
["lengths"]=>
NULL
["num_rows"]=>
int(1)
["type"]=>
int(0)
}
I am using similar to the following code to process query/result:
$result = mysqli_query ( $conn, $sql );
if ($result) {
var_dump($result);
if (preg_match('/^[[:space:]]*select/i', $sql) == 1) {
while ($row = mysqli_fetch_assoc ( $result )) {
$return_val[sizeof($return_val)] = $row;
}
if (!clear_results($result)) {
print "error: ".$sql;
}
}
} else {
http_response_code(401);
print $sql . "\n";
print mysqli_error($conn);
die("error");
}
function clear_results($result) {
global $conn;
if ($result) {
if (!mysqli_free_result ( $result )) {
return false;
}
}
while (mysqli_more_results($conn)) {
mysqli_next_result($conn);
mysqli_free_result(mysqli_store_result($conn));
}
return true;
}