Volume error

[pj909]

When I use this code on an index like $SPX, $NDX, etc. -- Volume(0) is returning a value other than 0 (zero). Historical bars are fine - only the current bar has a steadily increasing value. Works fine on stocks that have volume.

debugClear;
bInit = false;

function main() {
    if (!bInit) {
        xVol = volume();
        xMAV = sma(5, xVol );
        bInit=true;
    }
    nVol = xVol.getValue(0);
    nAvgVol = xMAV.getValue(0);
    debugPrintln(volume(0) + " " + nVol + " " + nAvgVol);
return;
}