Doctrine2 DQL and stored procedure issue - postgresql

Question

I need to execute with doctrine a query like this
SELECT convert_currency(p.amount) FROM policy p
convert_currency is a stored procedure I developed, waiting for a numeric argument.
I follow the DQL steps and create on CoreBundle/DQL the convertCurrency function:
<?php
namespace DeltaRM\CoreBundle\DQL;
use \Doctrine\ORM\Query\AST\Functions\FunctionNode;
use \Doctrine\ORM\Query\Lexer;
class ConvertCurrency extends FunctionNode
{
public $value = null;
/**
* Parse DQL Function
*
* #param Parser $parser
*/
public function parse(\Doctrine\ORM\Query\Parser $parser)
{
$parser->match(Lexer::T_IDENTIFIER);
$parser->match(Lexer::T_OPEN_PARENTHESIS);
$this->value = $parser->ArithmeticPrimary();
$parser->match(Lexer::T_CLOSE_PARENTHESIS);
}
/**
* Get SQL
*
* #param SqlWalker $sqlWalker
* #return string
*/
public function getSql(\Doctrine\ORM\Query\SqlWalker $sqlWalker)
{
return "convert_currency(" . $this->value->dispatch($sqlWalker) . ")";
}
and declare it on my config.yml
And on a repository, I use this function
$qBuilder = $this->getEntityManager()
->createQueryBuilder()
->select("convertCurrency (p.amount)")
->from("DeltaRMAssuranceBundle:Police", "p")
;
It works perfectly, the generated sql statement is
SELECT convert_currency(a0_.amount) AS sclr0 FROM police a0_
But I get an sql error while doctrine execute it :
SQLSTATE[42883]: Undefined function: 7 ERROR: function convert_currency(numeric) does not exist
If I execute directly on pgadmin the same sql, it works, the function is well known
If I do the exact same thing with max instead of convert_currency, it works perfectly.
What did I miss?