php - Trying to get property of non-object MySQLi result

Got a bit of PHP code I'm struggling with - had a search around Google etc. and tried everything mentioned, but for some reason I'm having trouble solving it.

The problem is:

I have some code that is querying a database for the presence of a particular user.

The code (it's a method inside a class)

global $mysqli;
// Query specified database for value
$q = 'SELECT id FROM ' . $database . ' WHERE username = \'' . $username . '\'';
$r = $mysqli->query($q);
var_dump($r);
if ($r->num_rows) {
    // If row found username exists so return false
    return false;
}

...
?>

I've var dumped the result of the query ($r) and got this:

object(mysqli_result)#4 (5) { ["current_field"]=> int(0) ["field_count"]=> int(1) ["lengths"]=> NULL ["num_rows"]=> int(1) ["type"]=> int(0) }

This is correct, there should only be 1 row as above.

I do get this error linking to the line saying if ($r->num_rows) {

Notice: Trying to get property of non-object in FILE on line LINE

but I don't know why since the object is valid (as above) and it should be working fine. From what I can tell it seems to be going through alright, I'm just wondering why there's an error. I'm sure it's something simple but I'd appreciate any help.