protected function _checkCurrencyId( $currencyid, $null = true )
	{
		if( $null === false && $currencyid === null ) {
			throw new MShop_Exception( sprintf( 'Invalid ISO currency code "%1$s"', '<null>' ) );
		}

		if( $currencyid !== null && preg_match( '/^[A-Z]{3}$/', $currencyid ) !== 1 ) {
			throw new MShop_Exception( sprintf( 'Invalid ISO currency code "%1$s"', $currencyid ) );
		}
	}