mysqli-trouble with mysql5

Hi;
I am using the mysqli-extension on all my database queries.
Everything worked just fine when I used MySQL 4, but when the database was
upgraded to MySQL 5 (5.0.22), I got a strange error.
Here is an example:

function example(){
global $table, $mysqli;
$min = 2;
$stmt = mysqli_prepare($mysqli, "SELECT SUM(Id) FROM $table WHERE
Id>?");
if ($stmt){
mysqli_bind_param($stmt, 'i', $min);
mysqli_execute($stmt);
mysqli_bind_result($stmt, $number);
mysqli_stmt_close($stmt);
}
else echo "failure";
}

This worked fine with mysql4, but with mysql5 i get a "proxy server error".

BUT: By removing SUM from the query, everything works fine:
$stmt = mysqli_prepare($mysqli, "SELECT Id FROM $table WHERE Id>?");

(but of course the query gives me not an interesting result).

SO: It seems that SUM messes things up, but there are noe good reasons for
it as I can see.
When I comment the line
// mysqli_bind_result($stmt, $number);
i do not get the error.

I use PHP 5.0.4, and MySQL 5.0.22. I use a tomcat server.

Any suggestions...?