Getting "-nan(ind)" when trying to generate random variates

I am trying generate random variates by trying to generate two standard normal variates r1, r2, by using polar coordinates along with a mean and sigma value. However when I run my code, I keep getting a "-nan(ind)" as my output.

What am I doing wrong here? The code is as follows:

static double saveNormal;
static int NumNormals = 0;
static double PI = 3.1415927;

double fRand(double fMin, double fMax)
{
    double f = (double)rand() / RAND_MAX;
    return fMin + f * (fMax - fMin);
}

static double normal(double r, double mean, double sigma) {
    double returnNormal;
    if (NumNormals == 0) {
        //to get next double value
        double r1 = fRand(0, 20);
        double r2 = fRand(0, 20);
        returnNormal = sqrt(-2 * log(r1)) * cos(2 * PI*r2);
        saveNormal = sqrt(-2 * log(r1)) * sin(2 * PI*r2);
    }
    else {
        NumNormals = 0;
        returnNormal = saveNormal;
    }
    return returnNormal*sigma + mean;
}