Help, Corner Analysis doesn't work in vco, error in evaluating the expression.

I'm Simulating a vco and want to get the Kvco and its corner analysis.

The Kvco itself is evaluated correctly but its corner analysis is not working.

The error that showed up to me is:

ERROR (ADE-1057): Could not evaluate expression
        Name:'Kvco'
        Expression:'deriv(harmonic(xval(getData("/net15" ?result "pss_fd")) '1))'
     because of the following error(s)
        ("error" 0 t nil ("*Error* drfGetWaveformYVec (DRL-18): argument not a valid srr waveform interface object nil"))
     Ensure that the definition of the expression is correct.

thanks.