parameters not working under PDO

It's quite possible that I'm missing something obvious here. The following code fragment does not return any rows, but if I take out the parameters and replace them with hardcoded strings (enclosed in single quotes), I get the right results. I've scattered "print" statements throughout, and the query seems to get past execute() OK, it's just not returning anything from the call to fetch() Am I missing something obvious? $stmt = $conn->prepare("select t.z from towns t

                        join counties c on t.county_z = c.z
                        where t.me = ?
                        and c.state_z = ?");

if ($stmt->execute($parts)) {

    while ($row = $stmt->fetch()) {
         $town_z = $row['z'];
    }

} else {

    print $stmt->errorCode();
    print_r($stmt->errorInfo());

} Thanks in advance!