Creating default object from empty value

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PHP

database

i am implementing PDO in my code . this is how i did it ,i am getting an error message on line 221 as ...HipHop Warning: Creating default object from empty value in /var/www/webApp/services/CurrencyService.php on line 221 . please check and verify where i am going wrong.

public function getExchangeRate($curr) {

$stmt = $this->vConnection->prepare("SELECT rate FROM $this->tablename where curr=?");
$this->throwExceptionOnError();

$stmt->bindparam(1, $curr);
$this->throwExceptionOnError();

$stmt->execute();
$this->throwExceptionOnError();
$stmt->bindColumn(1, $row->rate); //line 221

if($row = $stmt->fetch()) {
return $row;
} else {
return null;
}
}