Rate() returning NAN

//The following consts are defined:
//FINANCIAL_PRECISION=1.0e-08
//FINANCIAL_MAX_ITERATIONS=128


//Function by PHPExcel project;
protected function RATE($nper, $pmt, $pv, $fv = 0.0, $type = 0, $guess = 0.1) {
        $rate = $guess;
        if (abs($rate) < self::FINANCIAL_PRECISION) {
            $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;
        } else {
            $f = exp($nper * log(1 + $rate));
            $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;
        }
        $y0 = $pv + $pmt * $nper + $fv;
        $y1 = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;

        // find root by secant method
        $i  = $x0 = 0.0;
        $x1 = $rate;
        while ((abs($y0 - $y1) > self::FINANCIAL_PRECISION) && ($i < self::FINANCIAL_MAX_ITERATIONS)) {
            $rate = ($y1 * $x0 - $y0 * $x1) / ($y1 - $y0);
            $x0 = $x1;
            $x1 = $rate;

            if (abs($rate) < self::FINANCIAL_PRECISION) {
                $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;
            } else {
                $f = exp($nper * log(1 + $rate));
                $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;
            }

            $y0 = $y1;
            $y1 = $y;
            ++$i;
        }
        return $rate;
    }