mysql - prepared statement failing for "SELECT" command, using mysqli in PHP

I've been able to execute multiple prepared statements using mysqli (in PHP).
However, for some reason the prepared statements always encounter an error when I try running a "SELECT" command.
For example, the following line will succeed:

$stmt=$mysqli->prepare("UPDATE events SET category='m' WHERE id=(?)");

However, the following line will fail:

$stmt=$mysqli->prepare("SELECT * FROM events WHERE id=(?)");

When I say fail, I mean that the next three lines will return 1 for the UPDATE command (indicating that one row was changed)...

$stmt->bind_param('i',$id);
$stmt->execute();
echo $stmt->affected_rows;

The next three lines will return 0 for SELECT:

$stmt->bind_param('i',$id);
$stmt->execute();
echo $stmt->num_rows;

For the record, I'm aware that prepared statements aren't that efficient for a single SELECT - this question is mostly academic.